“Exhaust corroded but not leaking”
This describes almost every exhaust in Britain over about five years old. The four words that matter are the last three: not leaking. Until it is, this advisory costs you nothing.
Rust on the outside of the exhaust, with the system still sealed. An exhaust spends its life alternately red hot and soaking wet, hanging in the road spray, so external corrosion is what happens rather than what goes wrong.
Nothing to spend today. Start budgeting for the section that looks worst — usually the back box — within a year or two.
Rear silencer only. Mid sections and a full cat-back system cost more — the exhaust guide covers the whole range by section.
“Corroded” is not a quote for a full system. Exhausts are replaced section by section, and a rattling heat shield — which sounds identical from inside the car — costs nearly nothing to fix.
An exhaust carries hot, acidic gases away from the engine and hangs underneath the car in everything the road throws up. Condensation forms inside it on every cold start and sits there until the system gets hot enough to evaporate it, which on short journeys never happens. That is why exhausts rust from the inside as well as the outside, and why short-trip cars need them soonest.
The system is not one part
Understanding this is what separates a hundred-pound bill from a five-hundred-pound one, because an exhaust is replaced in sections and only the failed section needs doing.
The back box. The rear silencer, the part you can see from behind the car. It runs coolest, so condensation lingers in it longest, and it is the section that corrodes first on the overwhelming majority of cars. It is also the cheapest to replace.
The mid section. Sometimes with a second silencer, sometimes just pipe. Middle of the range in both position and price.
The front pipe and flexible joint. Closest to the engine, hottest, and rarely corroded — but the flexi joint fails from flexing rather than rust, which is a different failure at a different time.
The catalytic converter. Technically part of the system, priced entirely differently, and covered in our catalytic converter guide. A quote that folds the cat into an exhaust job needs explaining.
Which means the useful question is always which section. A tester writing this advisory has looked at a specific part of a specific system, and can say which. So can any garage with the car on a ramp.
How long you actually have
Often years. Surface corrosion on an exhaust progresses slowly, and the transition from rusty to blowing is usually gradual rather than sudden. Plenty of cars carry this advisory for their remaining life.
The signal is sound, and you cannot miss it. When a section finally perforates, the exhaust note changes — deeper, louder, often with a distinct rasp under acceleration. That change is the moment the advisory becomes a job, and it is obvious from the driver’s seat without anyone needing to tell you.
Two things accelerate the process. Short journeys, because the system never gets hot enough to drive out the condensation that corrodes it from within — a car doing four thousand town miles a year will need an exhaust before one doing fifteen thousand motorway miles. And winter salt, as with everything else underneath.
What does not accelerate it is worrying about it. Of all the advisories on a typical certificate, this is among the safest to note and move past. Our mileage check gives you your annual figure if you want to know which category of use you are in.
The point where an advisory becomes a failure
An exhaust fails the test when it leaks gas anywhere other than the tailpipe, when it is insecure or a mounting has failed, when a silencer is so deteriorated it no longer silences, or when part of it is missing. Corrosion by itself is not on that list — which is exactly why the advisory says “but not leaking”.
The test for a leak is practical. The tester runs the engine and listens, sometimes partly obstructing the tailpipe to raise the pressure in the system and make a small leak audible. A hole that whistles under pressure fails; a rusty but sealed section does not.
Noise is assessed too. An exhaust judged unreasonably loud compared with a similar vehicle with a correct system fails, which catches both a rotten silencer and an aftermarket system that is simply too loud.
Emissions are a separate matter and a leak upstream of the sensor can affect them, which is one of the few ways a corroded exhaust causes a second failure rather than one.
The free fix that sounds identical
Above the exhaust runs a thin metal heat shield, protecting the floor of the car from the heat below. It is spot-welded, it is thin, and those welds fail with age. When they do, the shield vibrates against the exhaust or the floor and produces a rattle or buzz that rises with engine speed.
From inside the car it sounds exactly like a failing exhaust. Same area, same kind of noise, same conditions. It is one of the most commonly misdiagnosed sounds on any vehicle.
The fix is usually free or nearly so. A loose shield can often be bent back into position, secured with a large washer and its original fixing, or in some cases removed. A garage with the car on a ramp identifies it in seconds by grabbing the shield and shaking it.
Which makes this the first question to ask if a rattle is what sent you looking, rather than the certificate: is that the exhaust, or is that the heat shield above it? Our symptom checker puts the heat shield at the top of that ladder for exactly this reason.
| Rear silencer, aftermarket | £65 |
| Fitting kit, clamps and gasket | £12 |
| Labour — 0.75 h × £65 | £49 |
| Typical total | ≈ £126 |
Inside the £90–£180 range for the rear section. A full cat-back system on the same car runs several times that — see the exhaust guide for the breakdown by section.
What it costs, when it eventually goes
£90–£180 for a back box fitted at an independent garage, which is where this advisory usually leads. Mid sections cost more, front pipes more again, and a complete cat-back system is a different order of expense entirely.
Aftermarket is the normal choice and a sound one. Exhaust sections from established manufacturers fit properly and last as long as the original in most cases, at a fraction of the dealer price. This is not a component where the genuine part earns its premium for most owners.
Stainless steel is worth considering on a car you are keeping. It costs more up front and typically outlasts a mild steel system by years, sometimes for the life of the vehicle. On a car you plan to sell within a couple of years the maths does not work; on one you intend to run into the ground it usually does.
What should not be on the quote: sections that are not corroded, and the catalytic converter unless it was separately assessed. If a full system is proposed, ask which sections were found to be failing and to see them.
Corroded exhaust advisory — your questions
Does surface rust on an exhaust actually matter?
Not until it perforates. An exhaust corrodes from the outside through road salt and from the inside through condensation, and external rust is close to universal on British cars over five years old. What matters is whether the system is still sealed, which is what the advisory wording confirms.
When does a corroded exhaust fail an MOT?
When it leaks gas from anywhere other than the tailpipe, when it is insecure or a mounting has failed, when a silencer no longer silences effectively, or when part of it is missing. Corrosion alone is not a failure — which is precisely why the advisory specifies that it is not leaking.
Can a corroded exhaust be repaired instead of replaced?
Small holes and split joints can often be welded, and that is far cheaper than a new section — particularly for a flexible joint. Bandage and paste repairs exist but are temporary and rarely survive a winter. A section that is generally thin and flaking is beyond patching.
Which exhaust section usually goes first?
The back box, almost always. It runs coolest, so acidic condensation sits in it longest before evaporating, and it corrodes from the inside as well as the outside. Cars used mainly for short journeys need rear silencers soonest, because the system never gets hot enough to dry out.
How long does an exhaust last?
Mild steel systems typically last eight to twelve years, and considerably less on cars doing short journeys or driven on heavily salted roads. Many modern cars have partly or wholly stainless systems that last the life of the vehicle, so check what yours has before budgeting.
Is a stainless steel exhaust worth the extra?
On a car you intend to keep, usually yes — it typically outlasts a mild steel replacement by years and often never needs doing again. On a car you plan to sell within a couple of years the extra rarely pays back, since the next owner gets the benefit.