MOT advisory explained
“Number plate lamp not working”
The only light on your car you never see working, and the only advisory on your certificate you can almost certainly fix yourself before the kettle boils.
One of the small lamps that illuminate your rear number plate has stopped working. Where a car has two and only one has failed, the plate is still lit, which is why this came back as an advisory rather than a failure.
Nothing has worn out and nothing is deteriorating. A filament has broken, or a contact has corroded, and the fix is a bulb costing less than a coffee.
Fitted at a garage. Doing it yourself is usually a matter of two screws and a bulb costing a few pounds — see below before you book anything.
If it has been on your certificate before, it is worth checking the holder rather than fitting a third bulb. A lamp that keeps failing is telling you about water getting in, not about bulbs.
A “number plate lamp not working” advisory means one of the lamps lighting the rear registration plate has failed, but another is still working so the plate remains illuminated and the car passes. A garage charges £10–£25; the bulb itself costs a few pounds. Of 1.2 million tracked lamp advisories, 14.4% later returned as a failure.
Why a small bulb is on the test at all
It looks like the most trivial item on the whole certificate, and in engineering terms it is. The reason it is tested has nothing to do with the car and everything to do with being identified: a registration plate that cannot be read after dark defeats the point of having one.
It is a legal requirement in its own right, not just an MOT item. The rear plate has to be lit when your lights are on, and that obligation applies on every night of the year rather than once a year in a test bay. A car with an unlit plate can be stopped for it.
Which puts it in unusual company on this site. Almost everything else in this section — pads, springs, bearings — is about whether the car works. This one is about whether the car can be identified, which is a different kind of rule entirely.
And it is the one light you have no way of noticing. A headlamp out shows up as a dark patch on the road. A brake light gets flashed at you. An indicator clicks faster on the dashboard. The plate lamp shines backwards and downwards onto a plate you cannot see from the driver’s seat, and it gives no signal of any kind when it stops.
Fixing it yourself, in about ten minutes
This is the only advisory in the whole cluster where the honest advice is that you probably do not need a garage. On most cars it is genuinely a two-screw job, and the part costs a few pounds.
- Find out which one has gone. Reverse up to a wall or a garage door after dark with the lights on, and look at the reflection. Two pools of light where there should be two, one where there should be two — that tells you what you are dealing with before you undo anything.
- Get the lens off. Almost always two small crosshead or Torx screws in the lens itself, or a lens that unclips with a small flat blade. Some cars have the assembly inside the boot lid trim instead, in which case the panel clips down. Work gently: the plastic is old and lives outdoors.
- Check the bulb and the holder together. Pull the bulb and look at the contacts behind it. A blackened filament is a simple bulb. Green or white powder on the contacts is corrosion, and a new bulb in a corroded holder fails again within months — clean it with a small brush or replace the holder.
- Match the bulb before you buy. The old one has a type marked on the metal base, commonly a small wedge type or a festoon. Take it into the shop rather than guessing, and buy a pair — the second one is on the same age and will follow the first before long.
Two things worth knowing before you start. Do not touch a new bulb’s glass with bare fingers if it is a halogen type, as the oil shortens its life. And if the lens itself is cracked or the seal has perished, water is getting in — a new bulb goes in a wet housing and the problem comes straight back.
When it stops being an advisory
The line is whether the plate is still lit. Where a car is fitted with two lamps and one has failed, the remaining lamp usually keeps the plate legible, and the tester records an advisory. When no lamp works and the plate is unlit, that is a failure.
Which is why this advisory converts more readily than its size suggests. There is no gradual middle ground — the second bulb does not dim, it simply goes at some point, and on that day the car has gone from advisory to failure with no warning in between.
The plate itself gets tested alongside the lamp. Characters that are wrong in spacing, font or size, a plate that is faded, cracked or obscured, or one that cannot be read from a reasonable distance all fail on their own terms. If the advisory came with a comment about the plate rather than the lamp, that is a different repair.
Retrofitted LED units are a common source of trouble here. Many are fine, but some produce a colour or a beam that does not properly light the plate, and some are bright enough to be counted as dazzling to the rear. A unit that came with the car is safe; one bought to look good is worth checking.
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 | Oil leak |
|---|---|---|---|
| Became a failure at a later test | 14.4% | 18.7% | 1.3% |
| Ignored, no action taken | 24.9% | 37.1% | 43.5% |
| Fixed | 60.7% | 44.2% | 55.1% |
| 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 the plate lamp alone.
One in seven of these comes back as a failure, which is a great deal for something you can put right for the price of a sandwich. A quarter are left alone entirely.
Every other conversion rate on this site describes something getting worse — a pad thinning, a bush loosening, rust spreading. A bulb does not do that. It works, and then one day it does not, and there is no state in between for a figure to track.
So when one in seven of these returns as a failure, that is not a component deteriorating over 379 days. It is a year in which nobody walked round to the back of the car and looked. The number is a measure of attention rather than of decay — the only one in the whole record that is.
Which is also why it is fixable by looking. Reverse up to a wall once, after dark, and you have done the entire diagnosis.
How the risk changes with the age of the car
Broken down by how old the car was when the advisory was written, the lamp group rises to a peak in the late teens and then eases back.
| 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.
Bulbs do not care how old a car is, but housings do. The rise across the first fifteen years is mostly seals hardening and holders corroding — a lamp mounted low at the back of a car spends its life in spray thrown off the road, and once water is getting in, bulbs stop lasting.
The rest of the rise is about how much attention a car gets. Older cars change hands more often and are less likely to be looked over between tests, and this is precisely the fault that only gets found by somebody looking.
What it costs
| Job | Price | Notes |
|---|---|---|
| Bulb replaced at a garage | £10–£25 | Ten minutes, usually while you wait |
| Bulb bought and fitted yourself | A few pounds | Buy a pair — the other is the same age |
| Corroded holder replaced | Modest | Worth doing if bulbs keep failing |
| Complete lamp unit | More, and rarely needed | Only where the lens is cracked or the seal has gone |
Independent garage prices. This is the cheapest item anywhere on an MOT certificate, and one of very few where the model of car makes almost no difference to the bill.
If you are booking a garage anyway, mention it and it usually costs nothing. The labour is a couple of minutes with the car already there, so bundling it with a service or another job removes the charge almost entirely.
Replace both at once even though only one failed. They were fitted on the same day, they have run the same hours, and the second is not far behind. Doing both now means you are not repeating this in six months — and it removes the situation where a single remaining lamp is all that stands between an advisory and a failure.
If bulbs keep going, stop buying bulbs. Repeated failures in the same lamp mean water is reaching the holder, and each new bulb is being fitted into the cause of the problem. Cleaning the contacts and replacing a perished seal costs very little and actually ends it.
Enter your reg — we’ll read your MOT history, decode each advisory and price it for your exact model.
What to do next
- Reverse up to a wall tonight, lights on. Thirty seconds, and it tells you whether one lamp has gone or both. That is the entire diagnosis, and it needs no tools.
- Look at the holder, not just the bulb. Green or white powder on the contacts means water has been getting in, and a new bulb in that holder will fail again. Clean it or replace it while the lens is off.
- Buy a pair and do both. They are the same age and have run the same hours. Replacing only the failed one means doing this again, and leaves you one bulb away from a failure.
- If the lens is cracked, replace the unit rather than the bulb. A cracked lens lets in the water that will keep killing bulbs, and no amount of replacing them addresses that.
Number plate lamp advisory FAQ
Will a number plate light failure fail an MOT?
It fails when the plate is not lit at all. Where a car has two lamps and only one has failed, the plate is usually still legible and the tester records an advisory instead. Because the second bulb does not dim before it goes, that advisory can become a failure overnight rather than gradually.
Can I change a number plate bulb myself?
On most cars, yes, in about ten minutes. It is usually two small screws in the lens, or a lens that unclips, and sometimes a panel inside the boot lid. Check the contacts behind the bulb while you are there — corrosion means a new bulb will fail again — and take the old bulb with you to match the type.
How much does it cost to fix a number plate light?
£10–£25 at a garage, or a few pounds if you buy the bulb and fit it yourself. It is the cheapest item that appears on an MOT certificate, and if the car is already in for something else most garages will do it for nothing because the labour is a couple of minutes.
Is it illegal to drive with a number plate light out?
The rear registration plate must be illuminated when your lights are on, so an unlit plate is an offence in its own right rather than only an MOT matter. Where one of two lamps still works and the plate remains readable, that is the situation an advisory describes.
Why does my number plate bulb keep blowing?
Almost always water. The lamp sits low at the back of the car in road spray, and once a seal has perished or a lens has cracked, moisture reaches the holder and corrodes the contacts. Each new bulb is then being fitted into the cause. Clean or replace the holder and seal rather than buying a third bulb.
Can I fit LED number plate lights?
Often, but check the result. Some retrofit units light the plate poorly, produce the wrong colour, or are bright enough to count as dazzling to the rear — any of which can fail a test. A unit fitted by the manufacturer is safe; one bought for appearance is worth looking at from behind the car before the MOT rather than after.
How do I know which of my number plate lights has gone?
Reverse up to a wall or a garage door after dark with the lights on and look at the reflection. Two pools of light where the plate is means both work; one means you have found it. It takes thirty seconds and saves guessing before you undo anything.
How often does a lamp advisory become an MOT failure?
Across 1.2 million tracked lamp advisories, 14.4% returned as a failure at a later test, 60.7% were fixed and 24.9% were ignored. For a bulb that figure means something unusual: nothing was deteriorating over those months, because bulbs do not fade. It measures the year in which nobody walked round to the back of the car and looked.