MOT advisory explained
“Headlamp aim too high or low”
The cheapest thing on your certificate, and one of the likeliest to be waiting for you as a failure next year. Nothing has worn out — a beam is simply pointing where it shouldn’t.
The dipped beam is pointing outside where it should, but not far enough outside for the tester to fail it. Nothing is broken and nothing is worn — the aim has drifted, and putting it back is a matter of turning an adjuster and measuring the result.
Worth doing at the first opportunity purely because it is trivial. This is the one item on the certificate where the repair costs less than the cup of coffee you have while waiting.
Both lamps, adjusted and checked on beam-setting equipment. Many garages do it for nothing alongside a service, and some include it with an MOT retest.
Check your levelling dial first. If the dashboard control is on 2 or 3 from the last time you carried a full load, the aim is wrong for a reason nobody needs to charge you for.
A “headlamp aim too high or low” advisory means the dipped beam is pointing outside the tested limits but not far enough to fail. Adjustment costs £20–£40 — the cheapest repair on the certificate. Of 1.2 million tracked lamp advisories, 14.4% later returned as a failure.
What the tester actually measured
A dipped headlamp does not throw light evenly. It projects a shaped pattern with a sharp horizontal edge along the top — the cut-off — which is what stops the light reaching oncoming drivers’ eyes. On a UK car that edge steps upwards on the nearside, so the beam reaches further along the kerb and the verge without lifting into the traffic coming the other way.
The test puts that pattern on a screen. Beam-setting equipment is wheeled up to each lamp, and the tester checks where the cut-off falls against marked limits. It is one of the few genuinely instrumented checks in the whole MOT — no judgement about how bad something looks, just a line that is either inside the box or outside it.
Too high is the common fault and the one that matters. A beam sitting above its limit is dazzling everyone you meet, and it does it without you noticing, because from the driver’s seat a high beam simply looks like good headlights.
Too low is the one drivers do notice, eventually. The road ahead goes dark sooner than it should, which feels like the bulbs are tired rather than the aim being wrong. If your lights seem to have faded over a couple of years, aim is worth checking before you buy anything.
Why aim drifts in the first place
Nothing on a headlamp wears out in a way that changes where it points. The aim moves because the car around it moves, and there are four ordinary ways that happens.
- Load. Weight in the boot or on a towbar squats the back of the car and lifts the front, and the beams lift with it. This is the most common cause by a distance, and it is also the only one with a control on the dashboard designed to correct it.
- Suspension settling. Springs sag slowly over years, and a car sitting lower at one end than it did when new points its lights differently. A tired or corroded spring at one corner can do it on one side only, which is why a lamp can be out on the nearside and perfect on the offside.
- A bulb changed carelessly. Some bulbs can only seat one way; others can be pushed home slightly out of position, and a filament a millimetre from where it should be throws the pattern off. If the advisory appeared the year after a bulb was replaced, that is the first thing to suspect.
- A knock, or a clouded lens. A minor front-end impact moves a lamp unit even where nothing looks damaged. And a lens that has gone hazy scatters the beam so the cut-off softens — at which point no amount of adjustment produces a clean edge, because there is no longer a clean edge to position.
Check your levelling dial before you pay anybody
Most cars have a small numbered wheel or set of buttons near the light switch, marked 0 to 3 or similar. It tilts the beams down to compensate for a loaded car, and it is the single most commonly misused control on a dashboard.
Zero is the setting for an ordinary load. Driver, maybe a passenger, nothing heavy in the boot. The higher numbers are for a full car, a loaded boot or a trailer, and they drop the beam progressively.
The trap is that people set it once and forget. Somebody moves house, winds it to 3, unloads the car and never touches it again — and from then on the lights are aimed at the road ten metres in front of the bumper. It also works the other way: a dial left on 0 with a heavy load lifts the beams into oncoming traffic.
Look at yours tonight, on an empty car. If it is not on the lowest setting, put it there and see whether the road ahead changes. That is a thirty-second check that occasionally solves the whole advisory, and it costs nothing.
Cars with automatic levelling do not have the dial — a sensor on the suspension adjusts the beams as the car squats. When those systems fail they usually park the lamps at one extreme, so the symptom is beams that are obviously wrong all the time rather than drifting slightly.
When it stops being an advisory
The limits are drawn on the beam tester, and there are two of them. Aim outside the wider set is an advisory; aim outside the tighter working limits is a failure. Because it is measured rather than judged, the same car gets the same answer at any station with working equipment — which is unusual on a certificate full of assessments.
That makes this one of the few advisories you cannot argue with. Unlike a worn brake disc, where “not seriously weakened” is one tester’s opinion, a beam is either inside the marked area or it is not. Nobody is being cautious or generous; the equipment simply reports where the light went.
Other lamp faults fail on their own terms. A lamp that does not work, a lens so damaged or clouded that the beam pattern breaks up, a colour that is wrong, or a pair that do not match — any of those is a failure regardless of where the aim sits.
Retrofitted LED bulbs in a halogen reflector are a common trap. The reflector was designed around the exact position and shape of a halogen filament, and an LED emitter in the same socket rarely reproduces it. The result is a scattered pattern with no clean cut-off, which is both dazzling and unaimable — and it fails the test whatever the packaging claimed.
How often does this advisory become a failure?
This can be measured rather than guessed. Following every lamp advisory forward to the same vehicle’s later tests, across 1.2 million of them, shows what actually happened.
| What happened next | Lamps | Suspension | Axles and bearings |
|---|---|---|---|
| Became a failure at a later test | 14.4% | 18.7% | 6.5% |
| Ignored, no action taken | 24.9% | 37.1% | 23.8% |
| Fixed | 60.7% | 44.2% | 69.7% |
| Median advisory to failure | 379 days — the next MOT | ||
From the DVSA MOT testing bulk dataset (Open Government Licence v3.0): 829.7 million tests and 145.1 million advisories tracked to each vehicle’s later tests, from 1,222,360 lamp and electrical advisories. Defect code 4.1 covers lamps and electrical equipment as a group, not headlamp aim alone.
This is the second highest conversion rate of any category we have measured, behind only suspension. Which is difficult to square with the price: a wheel bearing costs £120–£300 and converts at 6.5%. A set of discs and pads costs up to £460 and converts at 9.4%. This costs twenty pounds and converts at 14.4%.
Read across this site and an obvious story suggests itself: expensive jobs get postponed, cheap ones get done. The lamp figures demolish it. The cheapest repair on the certificate converts more often than almost anything else, and a quarter of these advisories are simply left.
What separates this one is that the owner receives no signal at all. A wheel bearing hums louder every week and gets repaired seven times in ten. A corroded mounting threatens the car itself and gets repaired three times in four. A misaimed beam does nothing to you whatsoever — if anything, a beam that is too high makes your own driving feel better lit.
It is also the only advisory on the certificate whose consequences land on somebody else. Everything else here — pads, springs, bearings, rust — is a risk you are carrying yourself. This one is carried by whoever you meet on an unlit road.
How the risk changes with the age of the car
Broken down by how old the car was when the advisory was written, the pattern is a rise and then a fall.
| Age of car at the advisory | Advisories | Became a failure |
|---|---|---|
| 2 years | 1,717 | 5.2% |
| 7 years | 9,702 | 7.4% |
| 12 years | 90,315 | 13.5% |
| 17 years | 128,087 | 15.5% |
| 22 years | 13,650 | 13.5% |
Same dataset, broken down by vehicle age at the time of the advisory. Figures beyond the mid-twenties rest on small numbers and are not shown. Contains public sector information licensed under the Open Government Licence v3.0.
The curve peaks around seventeen years and then eases off, which is the same shape as a wheel bearing rather than the endless climb of corrosion. That makes sense: aim is a state, not an accumulation. It drifts out and can be put back, and nothing about last year’s misalignment makes this year’s worse.
The rise up to that peak is mostly the car settling and the lenses ageing. Springs that have softened, mountings that have moved, and plastic lenses that have hazed enough to blur the cut-off — none of which is present on a car still in its first few years.
What it costs
| Job | Price | Notes |
|---|---|---|
| Headlamp aim adjustment | £20–£40 | Both lamps, set on beam equipment |
| Done alongside a service or retest | Often nothing | The equipment is already out |
| Levelling dial set correctly | Free | Do it yourself before booking anything |
| Headlamp unit replaced | Varies enormously | Halogen to adaptive LED is a wide gap |
Independent garage prices. This is one of very few jobs where the model barely matters — the work is the same on a small hatchback and a large estate. Unit replacement is the exception and depends entirely on the lamp fitted.
Ask for it while the car is in for something else. The beam tester takes a few minutes to set up and most of the cost is that setup, so bundling the adjustment with a service or another job is where the price falls to nothing.
Aiming by eye against a garage wall is not the same thing. It gets a beam roughly level, which is enough to notice a gross error, but the test limits are tighter than the eye can judge and the cut-off has to be positioned rather than just aimed forwards. If it has been flagged, have it set properly.
If the lens is hazed, adjustment will not fix the advisory. A clouded lens scatters light so the cut-off blurs, and a blurred edge cannot be placed inside the limits. Restoration kits genuinely help on lightly oxidised plastic; heavy crazing needs the unit replaced, and that is the one line on this page with real money attached.
Enter your reg — we’ll read your MOT history, decode each advisory and price it for your exact model.
Headlamp aim advisory FAQ
How much does headlamp alignment cost in the UK?
£20–£40 at an independent garage for both lamps, set on proper beam equipment. Many garages do it for nothing if the car is already in for a service or an MOT retest, because most of the cost is wheeling the tester up rather than the adjustment itself.
Can I adjust my headlights myself?
You can move them, but you cannot check them. The MOT limits are tighter than the eye can judge and the job is positioning a sharp cut-off rather than pointing the lamp forwards. What is worth doing yourself is the levelling dial by the light switch — if it has been left on a high setting since you last carried a load, that alone can be the problem.
Why do headlights go out of alignment?
The car moves rather than the lamp wearing. Weight in the boot or on a towbar lifts the front, springs settle over years, a bulb pushed home slightly out of position throws the pattern off, and a minor knock can shift a unit without visible damage. A hazed lens does something different again — it blurs the cut-off so there is no clean edge to aim.
Will a headlamp aim advisory fail my next MOT?
More often than most. Across 1.2 million tracked lamp advisories, 14.4% returned as a failure at a later test — the second highest rate of any category we have measured, despite being the cheapest thing on the certificate to put right.
What does the headlight levelling dial do?
It tilts both beams down to compensate for a loaded car, and it is usually a numbered wheel near the light switch. Zero is for an ordinary load; the higher numbers are for a full car or a trailer. Leaving it on a high setting after unloading aims the lights at the road just ahead of the bumper, which feels like tired bulbs rather than a setting.
Do LED bulbs in standard headlights fail an MOT?
Usually, yes. A halogen reflector is shaped around the exact position of a filament, and an LED emitter in the same socket rarely reproduces it — the result is scattered light with no clean cut-off, which is both dazzling and impossible to aim within the limits. The claim on the packaging does not change what the beam tester sees.
Is driving with badly aimed headlights dangerous?
It is, but mostly for other people, which is what makes this advisory unusual. A beam aimed too high dazzles oncoming drivers while making your own view seem better lit, so the person carrying the risk is not the person who could fix it. Aimed too low, the danger is yours: the road goes dark sooner than your stopping distance needs it to.
Can cloudy or yellowed headlights be aimed correctly?
Not reliably. A hazed lens scatters the light so the sharp horizontal cut-off softens, and an edge that is not sharp cannot be placed inside the test limits. Restoration kits work well on lightly oxidised plastic; heavy crazing means replacing the unit, which is the only expensive outcome on this advisory.