Almost one in three cars fails its MOT on the first attempt, but the majority of these failures are caused by issues that are easy to spot and cheap to fix. Here are the ten most common reasons cars fail — and what you can do about each one before your test.
1. Lights and Signalling (25% of failures)
Faulty bulbs are the single biggest cause of MOT failure. This includes headlights, tail lights, brake lights, indicators, number plate lights and fog lights. The fix is usually a bulb costing a few pounds.
DIY check: Ask someone to stand behind the car while you test every light. Don't forget the number plate light and reverse light.
2. Tyres (10% of failures)
Illegal tread depth, uneven wear, cuts, bulges or incorrect tyre sizes all result in a fail. The legal minimum tread depth is 1.6mm across the central three-quarters of the tyre.
DIY check: Use the 20p test — insert a 20p coin into the tread. If you can see the outer band of the coin, your tread is too low. Check all four tyres and the spare.
3. Suspension (10% of failures)
Worn shock absorbers, broken springs and loose components are common, especially on older cars or those driven on rough roads.
DIY check: Push down firmly on each corner of the car and release. It should bounce back once and settle. If it bounces repeatedly, the shock absorbers are worn.
4. Brakes (8% of failures)
Worn brake pads, corroded discs and fluid leaks can all cause a fail. Brakes are safety-critical, so the MOT tester will check them thoroughly.
DIY check: Listen for squealing or grinding when braking. Check that the car pulls up straight without pulling to one side. Look through the wheel spokes to visually inspect pad thickness.
5. Windscreen and Wipers
A chip larger than 10mm in the driver's swept area or 40mm anywhere else on the windscreen will fail. Worn or damaged wiper blades are also common failures.
DIY check: Inspect the windscreen carefully in daylight. Replace wiper blades if they're streaking or leaving smears.
6. Exhaust and Emissions
Excessive exhaust emissions, leaks or a noisy exhaust will result in a fail. Diesel cars are tested more strictly than petrol cars.
DIY check: Start the car from cold and check for excessive smoke. Look under the car for any obvious holes or rust in the exhaust system.
7. Number Plates
Plates must be clean, legible, properly spaced and correctly formatted. Faded, cracked or non-standard plates will fail.
DIY check: Give your plates a clean and check that the characters are all clearly readable from a distance.
8. Mirrors
Cracked, missing or loose mirrors will fail the MOT. You need at least two functioning mirrors, including the driver's door mirror.
DIY check: Inspect all mirrors for cracks and ensure they're firmly attached and adjustable.
9. Horn
A non-functioning horn is an automatic fail. It's one of the simplest checks to do yourself.
DIY check: Press the horn and make sure it produces a consistent, audible sound.
10. Seatbelts
Frayed, damaged or non-locking seatbelts will fail. The tester checks all belts fitted to the vehicle, including rear seats.
DIY check: Pull each seatbelt out fully and check for fraying or damage. Tug sharply to ensure the locking mechanism engages.
Save Yourself Time and Money
Most first-time MOT failures are caused by issues that cost under £20 to fix. Spending 15 minutes checking your car before the test could save you the cost and hassle of a retest. Use MOTChecker to view your full MOT history and see what advisories were raised at your last test — these are the areas most likely to cause a failure next time.