What a car service actually includes

What a service actually is

Start with the thing no service guide says out loud. The MOT has a published manual running to hundreds of pages, written by the DVSA, telling a tester exactly what to examine and exactly where the line sits. There is no such document for servicing. Not a shorter one, not a voluntary one — none.

Five things people mean by “a service”

When two people talk about servicing a car they are usually talking about different things, and only one of them is binding on anybody. Sorting them out takes two minutes and it changes how the rest of this page reads, because almost every argument between a driver and a garage comes from mixing them up.

What the law actually requires

There is no offence of failing to service a car. There is an offence of using one that is dangerous, and there is a duty that runs continuously rather than annually. Regulation 100 of the Construction and Use Regulations puts it in one sentence: a vehicle and all its parts and accessories shall at all times be in such condition that no danger is caused, or is likely to be caused, to anyone in the car or on the road.

Section 40A of the Road Traffic Act 1988 is the sharp end of the same idea. It makes it an offence to use, or to permit somebody else to use, a vehicle on a road when its condition creates a danger of injury to any person. Neither provision mentions oil changes, filters or intervals. They describe an outcome and leave the method entirely to you.

So what is a service for?

Strip away the menu and a service does three jobs, and only the first of them is genuinely mechanical. Understanding which job you are buying is what stops a service being either a waste of money or a false economy — and it explains why the same car can sensibly be serviced at very different intervals in different hands.

Common questions

Interim, full and major

Three tiers, sold everywhere, defined by nobody. The useful way to read them is not “how many checks you get” but a much shorter question: which items are physically renewed, and which are only looked at. Once you sort the list that way, the gap between the tiers turns out to be far narrower than the price gap suggests.

The difference nobody spells out

Sorting a typical service menu into three states — replaced, inspected, not covered — produces a result most drivers find hard to believe. An interim service renews five things. A full service renews six. The extra item you are paying for is the air filter; the rest of the difference is somebody looking at more of the car and writing down what they saw.

That is not an argument against a full service. Inspection is most of the value in servicing, and a thorough look at brakes, suspension and tyres by someone who does it daily is worth having. It is an argument against assuming that the price step between tiers buys a proportionate step in parts — it does not, and knowing that changes how you read a quote.

Every item, and what happens to it

The table below is a convention, not a standard. It reflects what UK menus typically promise at each tier — and menus differ, which is exactly why the comparison is worth having in front of you. Where your handbook says something different, the handbook wins: see section 03.

Compiled from typical UK service menus. There is no national standard for these tiers, so treat this as the common shape rather than a rule — some menus put the cabin filter or a brake fluid test into the full service, and a few include a fault-code read. “If due” means the item is replaced only where the schedule interval has been reached, which is why two major services on the same model can differ by more than a hundred pounds. Prices for each level are on the car service cost guide.

The point count is a marketing number

“A 70-point check” sounds like a specification and behaves like an advertisement. Nobody defines what one point is. Checking four tyres can be counted as one point or four; a set of lights can be one or eight. Two garages doing identical work can honestly advertise very different totals, and the higher number is not the better service.

The question that replaces it is short. Ask which items on the list are replaced and which are inspected, and you have converted an advertisement back into a specification. That is the same question that appears on your invoice afterwards — and section 14 is about reading it.

Which one your car actually needs

Mileage decides more than anything else. A car covering twenty thousand miles a year reaches the oil’s useful limit twice between annual services, which is the entire reason the interim tier exists. A car doing four thousand miles never gets there on mileage — but the oil is still ageing, and the annual service is the one that matters.

Age decides the rest. The items that separate a major service from a full one — brake fluid, spark plugs, fuel filter — are on multi-year intervals, so on a car kept long enough they come round regardless of how far it goes. Skipping the major tier does not skip the intervals; it just means nobody checked whether they had passed.

Common questions

The schedule versus the menu

Two documents describe servicing your car. One was written by the people who designed the engine and lives in the glovebox. The other was written by whoever runs the garage and lives on a whiteboard. Only the first was written about your car — and most drivers have never opened it.

Fixed or variable: establish this first

Every car is built to one of two philosophies, and everything else follows from which one you have. A fixed schedule states an interval in months and miles, whichever arrives first, and it does not care how you drive. A variable schedule lets the car decide, and the dashboard tells you when it has made up its mind.

Variable systems are doing real measurement, not counting down. BMW’s condition-based servicing derives the oil interval from a sensor watching oil condition, level and temperature, combined with engine load, fuel consumption, idle time and distance since the last change. Mercedes and Volkswagen work on the same principle under different names, and Honda’s system reports what is left as a percentage of oil life.

Which means a variable system can be wrong in one specific way. It assumes the oil that went in last time was the right one. Fill a car built for extended intervals with oil to the wrong approval and the display carries on counting exactly as before — the algorithm has no way to know. That is the failure mode this section exists to explain.

What your handbook calls it

The system has a name, and the name is on the dashboard or in the index of the handbook. Recognising it takes a minute and settles the question of whether a garage’s annual menu matches what your car is asking for.

System names are the manufacturers’ own, reproduced so you can recognise what your dashboard is showing you. Deliberately no intervals appear in this table: the actual figure depends on the engine and the model year, not the badge, and a number printed here would be wrong for some of the cars reading it. The handbook has yours.

The specification, not the viscosity

“5W-30” is not a specification. Viscosity describes how the oil flows cold and hot, and two oils sharing it can be built to entirely different standards. The part that matters is the approval — the manufacturer’s code confirming the oil was tested against what that engine needs, including its emissions equipment and, on some engines, its belt.

This is where the extra pound a litre goes, and it is not marketing. An approval carries the interval with it, which is why the same engine can have two legitimate service schedules depending on what is in the sump. Getting it wrong does not produce a warning light or a noise; it produces a shorter life for parts nobody will inspect until they fail. What the oil actually does, and how much of it your engine takes, is section 04.

Belt or chain is a question about the engine

Guides that publish a belt-or-chain column by model get it wrong roughly as often as the model has engines. A Fiesta with the 1.0 EcoBoost has a belt that runs in oil; an older 1.25 has a chain. Both are Fiestas. The same split runs through most of the best-selling cars in Britain, which is why the answer has to come from your engine code rather than your badge.

It matters here rather than only in section 10 for one reason: on engines where the belt runs in oil, the oil specification and the change interval are part of that belt’s life. It is the clearest case of a servicing decision reaching into a job that no service covers. If you are not sure which you have, the cambelt or chain checker answers it from your registration.

Reading your own schedule

Common questions

Oil and filter

This is the one item every level of service replaces, and on most bills it is the largest single line. It is also the least understood: oil does not wear out the way a brake pad does, and the reason it needs changing has almost nothing to do with it getting thinner.

What the oil is actually doing

Lubrication is only the most obvious of five jobs, and it is not the one that fails first. Oil also carries heat away from the pistons and bearings, holds combustion soot in suspension so it cannot settle into sludge, helps the piston rings seal against the bore, and lays down a sacrificial anti-wear film on surfaces that would otherwise touch metal to metal.

Four of those five depend on additives rather than on the oil itself. Detergents and dispersants keep soot floating; anti-wear compounds bond to hot metal; anti-oxidants slow the oil’s own breakdown; viscosity modifiers hold the grade steady between cold and hot. Every one of them is consumed in use, and none of them is replenished by topping up.

These are mechanisms rather than measurements, and no threshold is being claimed for any of them. The point of the table is the pattern: only the first line is about the oil, and the other four are about what has been dissolved in it.

Reading the numbers on the bottle

A grade like 5W-30 is two separate statements joined by a hyphen. The first number with its W describes how the oil flows when cold — winter, not weight — and it governs how quickly the pump can get oil to the top of the engine on a January morning. The second describes how the oil behaves once hot, referenced to a standard high temperature, and it governs whether a film survives between parts under load.

Most engine wear happens in the first seconds after start-up, before pressure reaches everything, which is why the cold number matters more than its position on the bottle suggests. The grading system itself is set by the SAE J300 classification, and it is deliberately silent about anything except viscosity — which is why it can never tell you whether an oil suits your engine. That is the job of the approval, covered in section 03.

Ash is the part that cannot be undone

If the car has a particulate filter, the oil is quietly part of that filter’s lifespan. Some of what burns in a diesel engine comes from oil making its way past the rings, and the metallic additives in ordinary oil leave behind an incombustible residue. Soot can be burned off in a regeneration; that residue cannot be burned off by anything.

Which is why the low-ash oils exist — the C sequences in the ACEA system, formulated with less of the metal-bearing additive chemistry precisely so that less of it ends up as permanent load in the filter. Fitting conventional oil to an engine specified for a low-ash grade does not fail anything immediately. It shortens the filter’s working life quietly, over years, and the bill lands as a DPF replacement that nobody connects to an oil change three services ago.

The filter, and the washer nobody mentions

The oil filter is the cheapest component on the invoice and one of the few where quality is measurable. Inside it there are two valves as well as a filter medium: a bypass that opens if the element clogs or the oil is too cold to pass through — the engine would rather have unfiltered oil than none — and, on most spin-on designs, an anti-drainback valve that keeps the galleries wet while the car is parked.

That second valve is where cheap filters betray themselves. A poor one lets the oil drain back into the sump overnight, so every cold start runs briefly on whatever film is left. It produces no symptom you can hear, which is precisely why it is a false economy that never gets attributed to anything.

The sump plug washer is a one-use part, designed to crush into shape and seal against an aluminium sump. Reusing it is the usual cause of the slow weep that turns up as an advisory two years later — and over-tightening to stop that weep is how sump threads get stripped, which is a repair out of all proportion to the part that caused it.

Why oil ages even when the car doesn’t

The car that barely moves is the difficult case, not the easy one. Every cold start puts unburnt fuel past the rings and condenses water in the crankcase. On a long run both boil off; on a short one they stay, diluting the oil and feeding the acids that use up the anti-oxidant additives. A car doing a few miles at a time is working its oil harder per mile than one doing motorway distances.

This is the mechanism behind the annual interval, and the answer to the most common objection to it — that the mileage since the last service barely registers. The additive package has been depleting the whole time, the acidity has been climbing, and neither of those is visible on the dipstick. Oil that looks clean can be finished, and oil that looks black can be doing its job perfectly: dark oil means the detergents are holding soot in suspension, which is what they are for.

Between services

Common questions

Air, cabin and fuel filters

Most drivers think of a car as having a filter. It has four, they protect four completely different things, and only one of them is protecting the person sitting in the car — which is, predictably, the one most often sold as an extra.

The air filter: what it does, and what it doesn’t

Its job is to stop abrasive dust reaching the inside of the engine, and it is very good at it. Grit drawn past a filter goes straight to the bores and rings, where it does exactly what grinding paste does, permanently. That is the failure worth preventing — and it comes from a damaged or badly seated filter far more often than from a dirty one.

The legend that a clogged air filter throttles a modern engine is mostly that. The engine management system measures what is actually flowing and fuels accordingly, so a tired filter costs a little efficiency rather than a dramatic slice of power. Anyone promising a noticeable performance gain from a new one is selling rather than explaining — the reason to change it is protection, not recovery.

One habit worth refusing: blowing it out with an airline. Compressed air applied from the dirty side drives contamination into the medium instead of removing it, and it can open up the pleats. A filter is a consumable priced accordingly; cleaning one is a false economy in the literal sense.

The cabin filter: the cheap part that impersonates an expensive fault

Everything the heater and air conditioning blow at you passes through this filter first. When it loads up, airflow drops at every setting, the windscreen takes noticeably longer to clear, and the car develops a musty smell as damp organic matter sits in a dark box and does what damp organic matter does.

The fuel filter: mostly a diesel item

Most petrol cars do not have a serviceable fuel filter at all. Theirs sits inside the tank as part of the pump assembly and is designed to last the life of the car, which is why “fuel filter” appears on a service menu and then quietly does not apply to half the vehicles reading it. On a diesel it is a genuine, separate, replaceable item — and a consequential one.

What it is protecting is the most expensive fuel system fitted to ordinary cars. A modern diesel injection system runs at extreme pressure through openings measured in fractions of a millimetre, and it relies on the fuel itself for lubrication. Water and fine particulates are what destroy it, which is why diesel filters usually include a water separator with a drain. The bill when that protection fails is not a filter — it is injectors, and often the high-pressure pump with them.

It is also the filter where labour varies most. Some are a cartridge in an accessible housing; others require the system to be primed and bled afterwards, which is a different length of job. That, rather than the price of the part, is why the same line item lands differently on two invoices.

Where each one sits in the tiers

Menu convention, not a standard — see section 02. The parts here are inexpensive in every case; what separates one invoice from another is access, which is why it is worth asking what the job involves rather than what the filter costs. Deliberately no intervals are given: those belong to your handbook, not to a menu.

What to ask for

Common questions

Brakes at service time

Brakes are inspected at every level of service, which makes them the item most often reported and least often reported usefully. There are exactly two numbers on a car that settle the question, and one of them is stamped on the part itself where anyone can read it.

“Brakes checked” is not a measurement

A brake report that says checked, satisfactory or OK has told you nothing you can act on. The whole value of having brakes looked at during a service is the forecast — how many months of ordinary driving are left before this becomes a job — and a forecast requires a number. Two garages can do identical work and only one of them writes it down.

There is a harder version of the same problem. A pad glanced at through the spokes of a wheel is not a measurement either, and on the majority of cars it is a look at the wrong pad. That is worth its own explanation, because it is the single most common reason brakes are found to be far worse than the last service suggested.

The pad you cannot see is usually the worse one

Most cars use a sliding caliper, which does its work by pushing one pad and pulling itself onto the other. The whole assembly has to move freely on two greased pins for that to happen evenly. When those pins stiffen — and road salt sees to it that they eventually do — the piston side keeps working and the other side stops retreating properly.

The result is a pair of pads at wildly different thicknesses on the same wheel. The one visible through the wheel is usually the outer, and on a sliding caliper it is frequently the healthier of the two. This is why a wheel has to come off for the check to mean anything, and why uneven wear between inner and outer is a finding in its own right rather than a curiosity.

Caught at a service, it is a clean and a re-grease of the slide pins. Left alone, it takes the disc with it and can end as a seized caliper — which is the same story as most of this page: a cheap intervention nobody was told about, and an expensive one nobody expected.

The disc carries its own minimum

Every brake disc has a minimum thickness cast or stamped into its hub face, usually alongside the words MIN TH. That figure is the manufacturer’s, it is specific to that part, and it is the only measurement on the whole car that a driver can walk up to and read for themselves. A technician measures the disc across its wear face with a gauge and compares the two.

Two things follow from that. The first is that a disc has an objective end point, unlike a pad, whose replacement point is a judgement. The second is that the figure sitting on the part is not the same as the standard the MOT applies — the test asks a different question about the same disc, which section 12 sets out in full.

The brake fluid test, and why it can read optimistically

Brake fluid absorbs water out of the air, steadily and by design of the chemistry rather than through any fault. Water in the fluid lowers the temperature at which it will boil, and boiled fluid puts vapour in a system built entirely on the fact that liquid does not compress. The pedal goes long exactly when you least want it to, and never at any other time — which is why nothing in normal driving warns you.

The service check is a meter dipped into the reservoir, and that is where the subtlety lives. Water migrates towards the lowest and hottest parts of the system, which are the calipers, so a reading taken at the top of the car is a reading from the best-case end of it. A reservoir result close to the limit is worse news than the same number would be if it had been taken at a bleed nipple.

None of which appears on an MOT, at any age or mileage — no part of the test measures fluid condition. It is one of the clearest examples of something only a service can find, and the fluids as a family are dealt with in section 08.

What a good brake report looks like

Every item in the middle column comes from work a garage has already done during a normal service — none of it is extra labour, and most invoicing software has a field for it. Asking is free. What the work costs when it becomes a job is on the brake pads and discs guide.

Common questions

Tyres, wheels and geometry

Tread depth belongs to the MOT and is covered in full in what the MOT actually checks. What a service does with tyres is everything the test does not: pressures set against a table most people have never read, an age nobody looks up, and the one upsell in this part of the bill that should never be sold without evidence.

The placard is a table, and almost nobody reads past the first row

Your car does not have a tyre pressure. It has a set of them, arranged by how heavily it is loaded. The placard on the driver’s door shut, the fuel flap or the handbook gives at least two conditions — ordinary use and fully laden — and often separate front and rear figures within each. The gap between the rows is not trivial, and it exists because a loaded car needs a different tyre than an empty one.

The row most people never use is the one for the journey where it matters most. A full boot, four adults and a motorway run is precisely the condition the second row was written for, and it is also the condition under which an underinflated tyre builds heat fastest. Setting pressures is a two-minute job at a service; asking which row was used costs nothing and is worth knowing.

The four digits that tell you how old the tyre is

Every tyre carries its date of manufacture as the last four digits of its DOT code: the week, then the year. On some tyres the complete code appears only on the inner sidewall, which is why drivers so often conclude their tyres are undated — they are looking at the wrong side of a marking that is facing the suspension.

Rubber ages whether or not it is used, and a tyre with generous tread can have spent years in sunlight on a car that barely moved. That matters most for the items nobody rotates through: a caravan, a second car, and above all the spare, which can be older than everything else on the vehicle by a decade.

Alignment: the upsell that should have to prove itself

Wheel alignment is on neither routine and is priced separately every time — which puts it in the same category as an air conditioning re-gas: a real job, frequently needed, and equally frequently sold to people who did not need it. The difference is that alignment leaves evidence, so it is one of the few upsells you can adjudicate on the spot.

Toe is the angle that is adjustable on essentially every car, and the one that destroys tyres fastest. Camber — the lean of the wheel — is fixed by design on many mass-market cars and cannot be corrected without parts, which is why “four-wheel alignment” and “front tracking” are different products rather than two names for one. If a quote does not say which angles are being set, it has not said what is being bought.

Wheels: torque, studs and the key in the glovebox

Wheel nuts have a specified tightness and it is not “as hard as the gun will go”. Over-tightening stretches studs and distorts the disc it is clamping, which is one of the quieter causes of a brake judder that appears shortly after tyre work. A garage using a torque wrench for the final tighten is doing the job properly; it takes seconds and is entirely visible from the waiting area.

The locking wheel nut key is the smallest object on this page with the biggest capacity to ruin a day. It lives in the glovebox, under the boot floor or with the jack, it rounds off with age, and its absence turns a roadside puncture into a recovery. A service is the natural moment to establish that yours exists and still fits — and if the key is missing, to find that out somewhere warm rather than at the side of a road.

The spare nobody checks

A spare wheel is invisible to routine care from both directions — the MOT does not examine it, and most service menus do not either, so it can sit under the boot floor for the life of the car losing pressure at its own quiet rate. The one occasion it is needed is the one occasion nobody has prepared for.

Cars supplied with a sealant kit instead of a wheel have the same problem in a different form: the canister has a shelf life printed on it, and an expired one is a bottle of the wrong answer. Either way, it is worth asking for the spare to be pressure-checked during the service, and worth knowing that the answer will usually be that nobody has looked at it in years. What tends to have been written down instead is the tyre nobody replaced — which is what our tyre advisory guide is about.

Common questions

Fluids

Five liquids, five different jobs, and one pattern running through all of them: the check carried out at a service measures a single property, and in most cases it is not the property that decides when the fluid is finished.

Coolant is mostly not about freezing

Antifreeze is the least important of its three jobs and the only one it is named after. Coolant also raises the boiling point so the system can run hot under pressure without vapour forming, and — the part that quietly matters most — it carries a package of corrosion inhibitors protecting aluminium castings, the radiator core and the seal in the water pump from the electrochemistry of dissimilar metals sitting in hot liquid.

Those inhibitors deplete, and nothing on a service checks whether they have. The instrument a garage dips in reads the freeze point, which is a measure of how much glycol is present. Glycol does not disappear; the additives protecting your engine do. A coolant that tests fine to a deeply negative number can have been chemically inert for years, and the bill arrives later as a corroded radiator or a weeping water pump.

Which is also why topping up with water is worse than it looks. Water restores the level and dilutes everything protective in one motion. A litre lost and replaced with tap water is a litre of protection removed — and a system that keeps needing topping up has a leak, which is the actual finding.

Brake fluid: the grades, and what they can be mixed with

The DOT numbers describe performance, not brand or interchangeability in the way most people assume. DOT 3, DOT 4 and DOT 5.1 are all glycol-based and mix with each other, with the higher grades offering higher boiling points; a car specified for DOT 4 is not harmed by DOT 5.1. DOT 5 is the exception and the trap — it is silicone-based, does not mix with any of them, and putting it into a system designed for glycol fluid is a repair, not a top-up.

All of the glycol grades absorb water, including the expensive ones, so paying for a higher specification buys headroom rather than immunity. That is why the item sits on a time-based interval rather than a mileage one, and why it appears at major level on most menus. What the service test actually reads, and why it flatters the system, is in section 06.

Gearbox oil and the words “sealed for life”

This is the one place on this page where there is no clean answer, and pretending otherwise would be dishonest. Many manufacturers publish no change interval for a gearbox, and “sealed for life” is doing a lot of work in that sentence: it means the design life of the unit, which is not the same thing as the life of the car it is bolted into.

A garage suggesting a change on a high-mileage automatic is not, by that fact, selling you something you do not need. Heat is what finishes automatic transmission fluid, and a car that has towed, sat in traffic or covered serious distance has cooked its fluid harder than the schedule assumed. But nobody can promise that changing it prevents a failure, and that promise is worth refusing when it is offered.

The myth in the other direction deserves the same treatment. A fluid change on a badly neglected box sometimes coincides with a failure, and the change gets the blame — but coincidence and cause are different things, and a box that fails a fortnight after a service was usually failing already. If the unit is showing symptoms, the question is no longer about oil; it is the one covered in gearbox repair costs.

The two that may not apply to your car at all

Power steering fluid is disappearing from the fleet. Electric assistance has replaced hydraulic racks on most new cars for years now, and an electrically assisted system has no fluid, no reservoir and nothing to check. When a menu lists it as an item, the honest answer for a great many vehicles is that it does not apply — which is the same trap as a fuel filter on a petrol car in section 05, arriving from a different direction.

AdBlue is not really a fluid in the servicing sense at all. It is a reagent consumed by the exhaust after-treatment on modern diesels, and it is closer to fuel than to oil: you buy it, it gets used up, and it needs replacing between services rather than at them. It crystallises around fittings and the filler neck, which is worth a wipe rather than a worry.

Running it dry has a consequence people find alarming and it is entirely by design. The car warns, keeps warning, and once the tank is empty it will not restart — the engine is prevented from running rather than broken. That behaviour is built into vehicles with selective catalytic reduction to stop the emissions system being bypassed, and no garage can switch it off for you.

Screen wash, briefly

The cheapest item on the list is the only fluid the MOT actually cares about, because washers that do not deliver are a test item in their own right. Filling the bottle with plain water is the common economy and it has two costs: it freezes in the pipework at exactly the time of year you need it, and standing water in a warm bottle grows the biological film responsible for the smell people blame on the air conditioning.

Common questions

Spark plugs and glow plugs

Two components with almost the same name, doing opposite jobs. One fires on every combustion cycle for the life of the engine and sits on a schedule. The other works for a few seconds a day, appears on no schedule anywhere, and introduces itself on the coldest morning of the year.

A tired spark plug shows itself under load

Spark plugs do not fail so much as gradually ask for more. Every firing erodes a little metal from the electrodes, so the gap the spark has to cross grows wider year by year, and the coil has to generate a higher voltage to bridge it. For a long time it succeeds, and nothing at all is noticeable.

Where it stops succeeding is the useful part. A spark jumps a gap most easily in thin air and least easily under pressure — so the first place it fails is the cylinder at its most compressed, which is the engine working hard. That is why the earliest honest symptom is a stumble or flat spot during firm acceleration, while the same car idles perfectly on the drive. Drivers tend to look for the opposite, and conclude nothing is wrong.

The electrode material is what sets the service life, not the brand on the box: fine-wire precious-metal electrodes erode far more slowly than plain ones, which is the whole reason they cost more and why the interval attached to them is longer. Which interval applies to your engine is a question for the handbook rather than a menu — section 03.

The chain that starts with the cheapest part on the car

Delaying plugs is not a saving that stays where you left it. A widened gap means a higher voltage demand every single firing, and the component being asked for that voltage is the ignition coil. Coils are not designed to spend years at the top of their range, and they fail — usually one at a time, usually inconveniently.

What follows can be considerably worse. A cylinder that misfires is pushing unburnt fuel into the exhaust, where it can burn inside the catalytic converter instead of in front of it. A converter is not built to be a combustion chamber, and sustained misfiring can cook it — which turns a modest set of plugs into a catalytic converter, several hundred pounds later.

None of that is a certainty, and it does not happen in a week. But the shape of it is the shape of most of this page: the cheapest item in the chain is the first one, and it is the only one anybody had the chance to plan for.

Glow plugs: the part with no schedule

A diesel has no spark, so cold starting depends on heat that the engine cannot yet make for itself. Glow plugs supply it — and they keep working after the engine has caught, warming the chamber for a while longer to keep combustion clean while everything is cold. That post-heating phase is why a failing set often shows up as smoke and clatter for the first minute rather than as a refusal to start.

They fail one at a time, which is what makes the symptom so misleading. With one plug dead, a warm engine on a mild day starts as if nothing has happened. The fault becomes visible only when the weather takes the margin away, so a problem that began months earlier presents as a car that has “suddenly got worse since it turned cold”. Nothing changed except the temperature.

Where each one actually sits

Menu convention, set out in full in section 02; the wider list of components that fall outside both routines is in section 11. Emissions readings on an MOT will eventually reflect an ignition problem, but by then the fault is old — the test measures the result, not the cause.

Common questions

Belts, chains and wet belts

The most expensive scheduled job on an ordinary car is the only one that appears on no level of service — and on no part of the MOT either. Nobody is tracking it except you, and most people do not know they have been given the job.

Why the biggest job is on none of the lists

A timing belt is not a maintenance item in the sense the rest of this page has been using. It is a scheduled repair: hours of labour, engine-specific tooling, and an interval that has nothing to do with the annual rhythm a service menu is built around. Nobody put it on the list because it does not fit the shape of the list.

The consequence is the part that matters. A service will not raise it, an MOT cannot see it, and the belt gives no symptom before it fails — so the only mechanism by which it ever gets done is somebody remembering. On a car that has changed hands once or twice, that somebody frequently does not exist. It is the clearest illustration on this page of what section 12 sets out: two routines, and a gap between them where the expensive things live.

The wet belt shifts the risk somewhere unexpected

A timing belt running inside the engine, immersed in oil, is now common on downsized petrol engines and on a good many diesels. It buys quietness and packaging, and it changes the failure mode. A dry belt in a housing has one way of ending badly. A wet belt has two.

The second one does not involve the belt breaking at all. As the material ages it sheds fine debris into the oil it is sitting in, and that debris goes wherever the oil goes — including to the strainer on the oil pump pickup. A partially blocked pickup starves the engine of pressure, and the damage that follows has nothing obviously to do with a belt. Engines have been written off by a component that was still, technically, going round.

Which is why oil specification and oil interval stop being a lubrication question on these engines and become part of the belt’s own survival. It is a genuine two-way dependency: the oil affects the belt, and the belt affects the oil. The costs and the engines involved are on the wet belt guide; what happens when a dry belt lets go is on the cambelt guide.

Chains: no interval is not the same as no end

A timing chain is not a scheduled item, and that is genuinely an advantage — there is no planned bill to budget for. But “no interval” has been quietly translated into “lasts forever”, and that is not what it means. Chains stretch as their pins and rollers wear, and the plastic guides and hydraulic tensioner that keep them under control wear alongside them.

The honest comparison is a different risk profile rather than a better one. A belt is a certain expense on a known date; a chain is an uncertain expense on an unknown one, and when it does arrive the labour is often greater. Neither is the superior choice — they are simply different shapes of cost, and it is worth knowing which shape you own.

The one warning a chain does give is worth taking seriously: a brief rattle on a cold start, before oil pressure reaches the tensioner, is the classic early sign that the slack has grown. It is covered properly in our guide to cold-start rattle, and it is the point at which asking is much cheaper than waiting.

Proof, or it hasn’t happened

Because nothing routine records a belt change, the paperwork is the only evidence there is. It takes one of three forms: an entry in the service book, an invoice naming the job, or a sticker under the bonnet with a date and a mileage on it — the habit of the better independents, and the reason to look before you assume.

With none of those, the working assumption has to be that it has not been done. That is not pessimism; it is the only assumption that costs less than being wrong. It applies with particular force to a car you have just bought, where a confident verbal “yes, that’s been done” is worth exactly nothing at the moment the belt disagrees. How service records work, and what a full history actually proves, is section 16.

Common questions

What a service does not include

A service is a checklist, not an examination of the car. Knowing which items are missing is useful; knowing why they are missing is more useful still, because it lets you work out where anything else belongs — including the part that has just started making a noise.

Things that do not wear out — they simply stop

This is the category people find hardest to accept, and it is the most honest thing on this page. A brake pad announces its remaining life in millimetres. An alternator does not. There is no dimension to measure, no percentage to report, and no meaningful difference between one that will run for another five years and one that will fail on Thursday.

So there is nothing a service could do about it even in principle. The alternator, the starter motor, the water pump, ignition coils and the fleet of sensors around a modern engine all live in this category: they work, and then they do not. A garage that told you an alternator had “about a year left” would be guessing, and a garage that says it cannot tell you is being straight with you.

What can be done is to take seriously the small signals these parts do produce. A battery that keeps needing a charge, a starter that hesitates when hot, a whine that changes with engine speed — these are worth mentioning at a service precisely because nothing on the sheet will find them. A mentioned symptom is worth more than an unmentioned one, and it costs the length of a sentence.

Things nobody can see without taking something apart

The second uncovered category is about access rather than physics. Gearbox internals, the state of a particulate filter, injector spray patterns, the play in a turbocharger shaft — every one of these could be assessed, and every one requires dismantling, specialist equipment or both. None of that fits inside a service, at any tier and at any price.

The practical consequence is that these components are diagnosed rather than inspected. Something happens first — a light, a noise, a loss of power — and only then does anyone look. That is not a failure of servicing; it is the boundary of what three hours and a ramp can establish about a sealed assembly.

Things deliberately kept off the list

The fourth category is commercial rather than technical. An air conditioning re-gas needs a dedicated machine and the best part of an hour. Wheel alignment needs a rig and a trained operator. Both are real work, priced separately because they are separate work — not because anyone is hiding them.

The one that surprises people most is diagnostics. A fault-code read is not part of a standard service on most menus, which means a car can be serviced from end to end with stored codes sitting in its modules that nobody looked at. If a warning light has been on, or has been coming and going, say so when you book — it converts a service into a service plus a question, and the question is the part that finds things.

So what does warn you?

For everything in categories two and three, you are the instrumentation. The car reports through sound, smell, feel and behaviour, and those reports are early — usually far earlier than the fault code that eventually confirms them. What follows is the translation table, and each row leads somewhere that goes into proper detail.

Every one of these is worth raising when you book rather than after the invoice is written — a symptom mentioned in advance can be looked at while the car is already on the ramp. Our symptom checker narrows most of them down in a minute, and the guides below take each one properly.

Common questions

What the MOT record shows about servicing

We track what happens to 145 million advisories after they are written. That record cannot tell you whether servicing works — nothing in Britain records which cars were serviced. What it can tell you is something more useful, and it is not what we expected to find.

What this record can and cannot show

Servicing is not recorded anywhere in Britain. There is no national database of who had a service and when — a car’s history is a paper trail held by its owner, not a public record. So no analysis, ours or anyone else’s, can compare serviced cars against unserviced ones. Anybody claiming to have done it is claiming access to data that does not exist.

What does exist is the other half of the equation. Every MOT writes down what a tester found, and we can follow each of those findings forward through later tests to see whether it turned into a failure, was fixed, or was still sitting there a year later. So we cannot measure cars. We can measure items — and we can sort those items by whether a service would have looked at them at all, using the same four-way split as section 12.

The finding we did not expect

Structural corrosion is the one category on this list that no service, at any tier, examines. It is also the category owners deal with most reliably: 75.9% of those advisories are resolved, and only 17.5% are still outstanding a year later — the best figures in the whole set by a clear margin.

Compare that with the two categories inspected on every single service. Suspension advisories are still unresolved a year later in 37.1% of cases; brakes in 36.1%. These are items a garage puts eyes on at every visit, writes down every time, and more than a third of them are still sitting there twelve months on.

So the thing that produces action is not being looked at. It is how the finding is expressed. A certificate that says corroded, structural lands on a driver as a verdict about the car’s integrity, and they act. A certificate that says brake pads wearing thin lands as a suggestion, and a third of the time nothing happens — which is precisely the argument section 06 makes about the difference between “brakes checked” and “front pads 4mm”.

Our analysis of the DVSA MOT testing dataset, 145,103,213 advisories followed forward across 216,057,619 pairs of consecutive tests. The middle column applies the service-coverage split from section 12. This is a comparison between categories rather than a weighted share of the whole record — see the note on method below. The same figures appear on what the MOT actually checks and in our guide to what MOT advisories mean. Contains public sector information licensed under the Open Government Licence v3.0.

Not every unfixed advisory is a mistake

Oil leaks are the most ignored category in the record — 43.5% still outstanding a year later — and ignoring them is largely correct. They are also the least likely thing on the list to end in a failure, at 1.3%. Drivers are not being reckless here; they are triaging accurately, treating a weep as something to keep an eye on rather than something to spend on.

That matters for how the rest of the table should be read. A high unresolved figure is not automatically negligence — it is only troubling where the same category also converts. Suspension is the case where both are true at once: the highest conversion in the set and one of the highest unresolved rates. That combination, not the unresolved figure alone, is what marks a category worth acting on.

The window is almost exactly one service

The median time between an advisory being written and the failure it becomes is between 376 and 384 days, and it barely moves across the eight categories. Corrosion, brakes, tyres, bearings and lamps degrade by completely different mechanisms on completely different timescales, and the record puts them all within a week of each other.

A year is one service interval. Which means that for most advisories there is exactly one scheduled appointment between the warning and the consequence — and it is the service. That appointment is the intervention point the whole system provides, and the unresolved figures suggest it mostly is not used as one. Nobody arrives at a service holding last year’s certificate.

A note on method

Two limits are worth stating plainly. First, nothing here compares serviced cars with unserviced ones, because no such record exists — every figure describes items, not vehicles or owners. Second, this is a comparison between eight categories, not a weighted share of the whole dataset: a category with a high conversion rate is not necessarily a large share of the record. The fuller methodology, and the full set of questions this page answers, is in the closing section.

Common questions

Service versus MOT

Put the two lists side by side and the overlap is smaller than almost anyone expects. Of thirty-nine things worth checking on an ordinary car, eleven appear on both routines, ten belong to the MOT alone, eight to the service alone — and ten are on neither list, at any price, ever. That last group contains the most expensive jobs a car can present you with.

The same item, two different questions

Where both routines look at the same part, they are not doing the same job. The MOT asks one question: is this dangerous, or illegal, today. A service asks a different one: how much of this is left. An item can be perfectly acceptable to a tester and three months from needing replacement, and both assessments are correct.

Brakes are the clearest case. The inspection manual does not give a tester a minimum pad thickness to work to for a pass — it asks whether the brake is significantly and obviously worn, and it says in terms that a disc worn below the manufacturer’s minimum is not, by itself, a reason for rejection. A service measures the pad in millimetres and tells you how many months you have. One is a legal threshold; the other is a forecast, and drivers routinely mistake the first for the second.

Where every item falls

MOT criteria from the DVSA MOT inspection manual for private passenger and light commercial vehicles, which applies in England, Scotland and Wales. Service coverage reflects typical UK menus, which have no national standard — the manufacturer’s schedule takes precedence over both columns where the two disagree. Contains public sector information licensed under the Open Government Licence v3.0.

The gap where the money is

The bottom-left box is the whole reason this page exists. These are jobs nobody inspects on a routine, which means the only warning you get is the one the car gives you — a noise, a smell, a change in how it drives. Knowing what lives there is what turns a nasty surprise into a planned expense.

Estimates for independent garages; main dealers typically charge around 50% more. Final quotes come from garages. Your car’s MOT history is the cheapest early-warning system available for the items that are recorded — the advisory checker translates what has already been written on your certificates, and section 13 sets out what those records show about servicing.

Booking the two together is a separate question — one of cost and sequence rather than coverage, and it has its own guide in full service and MOT cost. What matters here is that combining them does not close the gap. Two routines that skip the same ten items still skip them when you buy them on the same day.

Common questions

Reading the service menu

A price on a board is not a price for your car. It is the starting point of one, and the difference between the two is made up of items that are neither hidden nor dishonest — they are simply outside the number, waiting to be added once somebody has looked underneath.

What is inside the advertised number

A headline service price generally contains three things: an amount of labour, a quantity of oil, and a filter. Everything else on the checklist is inspection, which costs time rather than parts, and inspection is what the labour figure is buying. That is a coherent product and there is nothing wrong with it.

The gap opens where inspection turns into findings. A service is the one appointment where somebody looks at the whole car with the wheels off the ground, so it is also the appointment where things get found — and the things that get found are priced separately, because until the car was on the ramp nobody knew they existed. A low headline price is not a trick; it is a product whose final cost is decided after it has started.

The three verbs, and why they should never be blurred

Everything on a service list happens in one of three ways: something is checked, something is topped up, or something is replaced. The tiers in section 02 are built entirely out of which verb applies to which item, and the difference between them is most of the difference between two quotes.

Menus blur the three constantly, and invoices should not. “Air filter” on a list tells you nothing; “air filter — replaced” and “air filter — checked” are different products at different prices. The same goes for fluids, where topped up and changed are separated by an entire job. When comparing two quotes, the useful exercise is not comparing totals — it is writing the verbs next to the items and seeing where they differ.

Two clauses worth reading twice

“Up to” is the first. Oil is quoted to a stated quantity because engines differ enormously in how much they hold — section 04 covers the range — and an engine that takes more is charged for more. That is fair, and it is also why a single advertised price cannot mean the same thing for a small hatchback and a large diesel. Ask what your engine actually takes before comparing prices; it is one of the few numbers a garage can tell you over the phone.

“Parts if required” is the second, and it is the open-ended one. Nobody can quote for what has not been found yet, so the phrase is unavoidable — but as written it authorises an unknown amount of work. The fix takes one sentence at the booking: give a figure above which you want to be called. It converts an open clause into a closed one and it is entirely normal to ask for; a garage that resists it has told you something useful.

The extras, by decision rather than by price

Every service generates a conversation about additional work, and the useful way to sort it is not by cost but by what triggers it. Three categories cover almost everything offered at the desk, and knowing which one an item belongs to answers the question faster than any price comparison.

Prices for each of these sit on the car service cost guide; what matters here is the trigger rather than the figure. The pattern is consistent: items driven by an interval are worth saying yes to when the interval has arrived, items driven by a symptom are worth saying yes to when the symptom exists, and items driven by neither are worth a polite no. If a quote for work found on the ramp looks high, our repair quote checker compares it against typical UK prices before you agree to it.

Free MOTs, and what the law actually gives you

A free or heavily discounted MOT alongside a service is a discount, not a gift — the test fee is capped by statute, so the cost of doing it has to come out of the service margin somewhere. That does not make it a bad deal; it makes it a bundle, and bundles are worth comparing as bundles. The economics of the package are in full service and MOT cost.

Underneath all of this sits the Consumer Rights Act 2015, which requires a service to be carried out with reasonable care and skill and, where no price was agreed in advance, for a reasonable price. That is a genuine protection and it is also a slow one — invoking it means a dispute. The authorisation limit you set at the booking is the fast version of the same protection, which is why it is worth thirty seconds.

Common questions

Where to have it done, and the warranty question

One belief keeps more drivers in a dealer service department than any other: that going anywhere else forfeits the warranty. It is worth knowing exactly what a warranty claim is judged against, because the answer is three conditions long and none of them is the name of a business.

The rule that makes independent servicing possible

An independent garage can only service your car to the manufacturer’s schedule if it can get at the manufacturer’s information, tools and parts. That access is not a courtesy — it is the subject of a specific piece of UK competition law, the Competition Act 1998 (Motor Vehicle Agreements Block Exemption) Order 2023, in force since June 2023 and running until the end of May 2029.

What it does is prohibit certain restrictions in the agreements manufacturers make with their own networks. Suppliers cannot restrict independent operators’ access to the information, tools or training given to authorised repairers, and must provide it in a way that does not put the independent at a disadvantage; nor can they block the supply of spare parts to independent repairers. That is why an independent workshop can look up your schedule, buy the right parts and stamp the book.

It is competition law about agreements, not consumer law about warranties, and it is worth being precise about the difference. The Order is the reason an independent can do the work properly. Whether your warranty survives is a question about your warranty — which is a contract, and contracts are read rather than assumed.

Three products, one word

“Warranty” gets used for three different things, and the differences decide who pays. Sorting them out before you need one is considerably cheaper than sorting them out afterwards, and the second of the three is where most unpleasant surprises live.

Policy wording varies enormously between providers, so this describes the pattern rather than any particular product — yours is the one that counts, and it is worth reading the servicing clause before the next service rather than after a claim.

Matching quality, not matching logo

The parts condition is about specification, not branding. A great many parts fitted by dealers are made by the same manufacturers that supply the aftermarket, in the same factories, to the same drawings — the difference is frequently the box. What matters is that a part meets the quality and specification the vehicle requires, and that the invoice says so.

Which is where the habit from section 03 pays for itself. An invoice recording an oil approval code, a part number and a specification is evidence; one recording “oil and filter” is a receipt. The extra effort is a line of typing at the time and the difference between a claim and an argument later.

So where should you go?

That is a different question, and it has its own guide — independent garage versus main dealer compares labour rates, equipment and what each is genuinely better at. The short version is that the warranty issue is not the deciding factor most people assume, so the decision comes down to the ordinary ones: price, convenience and whether you trust the people.

There are real cases where the dealer is the right answer, and pretending otherwise would be as unhelpful as the myth this section exists to correct. Outstanding recalls are carried out by franchised dealers. Software updates for a specific known fault often are too. And if a warranty claim is already contested, having the servicing history sitting in the manufacturer’s own system removes one argument from a conversation that has enough of them.

Common questions

Service history

The least mechanical part of servicing is often the most valuable, and it is the part most likely to be misunderstood. A stamp is not evidence of work. An invoice is. And there is one layer of a car’s history that no owner and no seller can edit, which makes it the most useful of the lot.

The ladder of proof

Every claim about a car’s past sits on one of four rungs, and they are not close to equivalent. Knowing which rung you are being offered is most of what separates a confident purchase from a hopeful one — and it applies just as much to your own car, where the rung you are standing on is the one you built by what you kept.

A stamp says who, an invoice says what

The service book was designed for a world with less paperwork in it, and it has aged badly. A stamp confirms that a business saw the car on a date. It does not say which tier was bought, which items were replaced rather than inspected, which oil went in, or whether anything found was dealt with. Two cars with identically stamped books can have had entirely different amounts of money spent on them.

The invoice is the document that carries the meaning, which is why section 14 spends so long on what should be on it. Keep them and the book becomes a summary of something rather than a substitute for it. Throw them away and you are left holding a set of signatures on an unspecified claim.

The book is quietly being replaced

Most manufacturers now hold service records in their own systems rather than in the glovebox. An authorised repairer writes the visit into the maker’s database; independents may have access to do the same, and may not, depending on the marque and the arrangement. The paper book still exists, but on many newer cars it is no longer where the truth lives.

That produces a mismatch nobody warns buyers about. A car can have a complete digital history and an empty book, or a beautifully stamped book with nothing behind it in the system. Buyers look at the book, because it is the thing they can hold. It is worth asking a seller whether the history is digital, and worth asking a garage — before the work, not after — whether they will be recording it in the manufacturer’s system, in the book, or both.

The record nobody can edit

Every MOT a car has taken is in a public record, free to look up, with the mileage recorded at each test. That last detail is what makes it powerful: it is an independent, dated sequence of odometer readings, written by testers who had no interest in the car’s resale value and cannot be revised afterwards by anyone who does.

It corroborates or contradicts everything on the first three rungs. A history claiming regular servicing at eight thousand miles a year sits badly against MOT readings showing twenty. A mileage that fails to advance and then leaps is worth an explanation. And the advisories recorded at each test tell you what the car needed and, by their reappearance or absence next time, whether anyone dealt with it.

Our tools read that record for you. The mileage check plots the readings and flags discontinuities; the used car check pulls the history together before you buy; and our guide to reading an MOT history covers what to look at, in what order, and what a clean record does not prove.

Rebuilding a history that has gone missing

A lost book is not a lost history, and it is worth an hour of work. Garages keep records of what they invoiced, usually for years, and most will reissue copies for a car they worked on if you can tell them roughly when. A franchised dealer can normally check whether anything sits in the manufacturer’s system under that registration.

Start from the MOT record and work outwards, because it gives you the dates and mileages to ask about. Even a partial reconstruction changes what you are holding: three invoices and a public mileage trail beats an unstamped book and an assurance, and it beats it at the point where somebody is deciding what to pay you. Whether the timing belt was ever done is the one gap this exercise most often exposes — see section 10.

Common questions

Before you book

Everything on this page reduces to eight questions and four things to have ready. None of them requires technical knowledge, none takes more than a sentence, and all of them are about work a garage is doing anyway — the difference is whether you get told the results.

Have these four ready

The eight questions

They are gathered here in the order they come up, with a line on what each one is for. Nothing in this list asks a garage to do additional work — every answer already exists by the time the job is finished, and most of them exist on the technician’s sheet before the car comes off the ramp.

A garage that answers these easily is telling you something, and so is one that finds them awkward. None of them is a challenge — they are requests for information that has already been generated, and most workshops are pleased to be asked, because it is the part of the job that distinguishes them from a menu price.

Keep two things

The itemised invoice, and the MOT certificate. Between them they establish what was done, what it was done to, and what the state recorded about the car in the same period — which is the whole of a service history that anybody else can verify. Everything in section 16 follows from whether these two documents survive.

Photograph both. Paper goes missing during the years when it matters most, and a photograph on a phone is a record that outlives a glovebox. It is the cheapest thing on this page and the one most likely to be worth money later.

Common questions

Free tools that answer the rest

Several of the questions above have a shortcut. These are ours, they are free, and none of them asks you to sign up for anything.