What fails an MOT? The 10 most common faults, from 30 million tests
The faults that fail the most cars, how many of them you can spot yourself in ten minutes, and what to check before your test.
By the CarComparison team · Updated · How we research
In 2024, 28.5% of cars aged 3 to 20 failed their MOT first time. We went through all 30,771,766 of those tests to see what they failed on. The answer is reassuring if your test is coming up: the single most common reasons are things you can spot on your drive in ten minutes.
The 10 most common reasons cars fail
Each figure is the share of all tests that failed for that reason, so a car can appear under more than one. 6 of the top 10 are things you can check, and often fix, yourself.
| # | Fault | Share of tests | Check it yourself? |
|---|---|---|---|
| 1 | A sidelight or rear light not working | 2.5% | Yes |
| 2 | Tyre tread below the 1.6mm legal limit | 2.5% | Yes |
| 3 | A broken suspension spring | 2.1% | Garage |
| 4 | Wipers that don't clear the windscreen | 2.1% | Yes |
| 5 | Brake pads worn below 1.5mm | 2.0% | Garage |
| 6 | A brake light not working | 1.9% | Yes |
| 7 | A worn suspension or steering ball joint | 1.8% | Garage |
| 8 | An engine or system warning light on | 1.5% | Yes |
| 9 | Worn suspension pins or bushes | 1.5% | Garage |
| 10 | A headlight not working on dipped beam | 1.5% | Yes |
Which parts of the car fail most
Grouping every fault by the part of the car it's in shows where the problems really are:
- Lights and electrics: 11.0% of tests failed on at least one fault here.
- Suspension: 8.8% of tests failed on at least one fault here.
- Brakes: 6.7% of tests failed on at least one fault here.
- Tyres: 6.6% of tests failed on at least one fault here.
- Visibility: 4.6% of tests failed on at least one fault here.
- Body and structure: 3.3% of tests failed on at least one fault here.
- Exhaust and emissions: 3.1% of tests failed on at least one fault here.
Lights and electrics top the list, ahead of suspension and brakes. Most lighting failures are a blown bulb, which is the cheapest repair there is, but at the test centre it still means a failed certificate and a retest.
A ten-minute check before your MOT
These cover the most common faults you can find without tools:
- A sidelight or rear light not working. Walk round the car with the sidelights on. Bulbs are often a few pounds.
- Tyre tread below the 1.6mm legal limit. Put a 20p coin in the main grooves: if you can see its outer band, the tread is close to the limit.
- Wipers that don't clear the windscreen. Replace worn or smearing blades, and top up the washer fluid: an empty washer bottle can fail too.
- A brake light not working. Ask someone to watch while you press the brake, or reverse up to a window and look.
- An engine or system warning light on. If a warning light is on, get the fault read and fixed before the test: it's an automatic fail.
- A headlight not working on dipped beam. Check dipped and main beam against a wall or garage door.
- Number plates and screen. Plates must be clean and readable, and chips in the driver's view can fail if they're larger than 10mm.
Anything that needs the car on a ramp, like springs, ball joints and brake pads, is worth leaving to a garage. If you hear knocking over bumps or squealing when you brake, mention it when you book the MOT. Many garages will quote for the work at the same time.
Does the model matter?
A lot. Some models fail far more often than average on one particular part. For example, our MOT reliability ratings show which faults each model fails on most compared with the average car of the same age. If you're buying used, it's worth checking whether the car you're looking at has a known weak spot, and our free car check shows a specific car's MOT history and advisories from its number plate.
Based on 31 million MOT tests of cars aged 3 to 20 in 2024, from DVSA's published data.