MOT Failure Reasons Explained: What They Mean When Buying a Used Car
A car's MOT history shows every pass, every failure, and every reason the car was failed. For anyone buying used, that failure record is one of the most useful things you can look at.
A single failure for a blown bulb is nothing. Repeated failures for brakes or suspension are a different story entirely. This guide breaks down the most common failure reasons, what each one costs to fix, and what it tells you about how the car was looked after.
What happens when a car fails its MOT
When a car fails, each problem is logged against one of several categories:
- Minor defect: Does not fail the car but is noted (similar to an advisory)
- Major defect: The car fails. Must be repaired before the car can be legally driven
- Dangerous defect: The car fails and should not be driven at all until repaired
The full list of failures and the reasons behind them are stored on the DVSA database and available to check for free. When you're buying a car, look at how many times it has failed, what it failed on, and whether the same issue keeps coming back.
The most common MOT failure reasons
Lights and electrical
The most common reason for an MOT failure in the UK. A blown bulb is a £5-20 fix. On its own, it tells you very little about the car. Where it gets more interesting is if the same bulb or electrical issue keeps failing year after year, which can point to a wiring fault that's never been properly traced.
The headlights are pointing in the wrong direction. Cheap to adjust at most garages. Usually a sign of a minor knock or poor previous repair rather than anything structural.
Tyres
Tyres must have at least 1.6mm of tread across the central three-quarters of the tyre. A failure here means the car was being driven on illegal rubber. Budget £80-150 per tyre. If this appears repeatedly in the history, the car has been regularly driven in an unroadworthy state.
A cut, bulge, or the wrong tyre type fitted. A one-off failure is not a major concern if it has been fixed. A pattern of tyre-related failures can suggest the car has been driven hard or the owner has been cutting corners on maintenance costs.
Brakes
The brakes were tested on a rolling road and didn't generate enough stopping force. This could mean worn pads or discs, seized calipers, or a brake fluid issue. Budget for a full brake inspection. If this failure appears more than once, the car has a recurring brake problem that has not been properly resolved.
The brakes are pulling harder on one side than the other. This affects stopping distances and vehicle control in an emergency. Causes include a seized caliper, contaminated pads, or uneven wear. A car that has failed on brake imbalance more than once has had a persistent braking problem. Get an independent inspection before buying.
Steering and suspension
Worn bushes, corroded springs, loose ball joints, or failed shock absorbers. One failure on an older car is not unusual. Repeated failures, or failures on multiple components at once, suggest the car has been driven hard and suspension maintenance has been neglected. Get quotes for the specific parts before agreeing a price.
There is excessive play in the steering wheel or a component in the steering system has failed. Steering failures are serious and should not be ignored. If this appears in the history more than once, or alongside suspension failures, consider carefully whether the car has been in an accident that was never properly declared or repaired.
Emissions
High CO or HC emissions usually mean a worn engine, a failing catalytic converter, or a fuelling issue. A one-off failure that was fixed is not worrying. A car that fails emissions repeatedly has an underlying engine problem that keeps coming back. Factor in the cost of a cat replacement (£200-600 on most cars) if this appears in the recent history.
A diesel smoking beyond the permitted level at the test points to a blocked DPF (diesel particulate filter), injector issues, or engine wear. DPF replacement is expensive, often £1,000 or more. A failure here on a diesel car is worth investigating carefully before buying.
Bodywork and structure
Rust on body panels is cosmetic. Rust that has reached a structural component, a chassis rail, a floor pan, or a subframe mounting point, is a different matter. A failure on structural corrosion means the car was dangerous to drive at the time of testing. Repairs can range from manageable to uneconomical depending on severity. Always inspect underneath the car in person.
A chip or crack in the driver's line of sight fails the MOT. Windscreen replacement is typically £150-400 depending on the car. On its own, it's not a red flag. Check the current windscreen at the viewing to confirm it has been replaced if this appeared on a recent test.
Seatbelts
A seatbelt that doesn't latch, retract, or hold correctly is a dangerous defect. This failure is uncommon and when it appears it should have been fixed before the car was retested. If a car shows a history of seatbelt failures, check every belt physically at the viewing.
What failure history tells you about the car
One failure on an older car for a cheap, easy-to-fix item is entirely normal. Look for these patterns instead:
Worth taking seriously:
- The same failure appearing on two or more consecutive tests
- Failures across multiple systems at once (brakes and suspension together, for example)
- A major failure in the recent history with no corresponding retest or service record
- The car failing and then being tested at a different garage on the same day (sometimes used to shop for a pass)
Less concerning:
- A single historic failure that was fixed and hasn't recurred
- Failures only on consumables (bulbs, tyres, wipers) with clean records on mechanical systems
- An older car with a couple of advisory-level items that were resolved
Check failure history before you view
The full MOT failure record for any car is publicly available through the DVSA. Checking it manually means copying the plate and searching the government site separately for every car you look at.
Don't Buy A Lemon does this automatically as you browse, showing the full pass, fail, and advisory history on every listing without switching tabs.
Frequently asked questions
Can you drive a car that has failed its MOT? It depends on the failure type. A major defect means the car must be repaired before it's road legal. A dangerous defect means it should not be driven at all. You can drive to a pre-booked repair appointment, but not otherwise.
How long does a seller have to fix a failed MOT? There is no set time limit, but the car cannot be used on public roads with a major or dangerous defect until it passes a retest.
Should I buy a car that has failed an MOT in the past? It depends entirely on what it failed on and whether it was fixed. A historic failure for a blown bulb means nothing. Repeated failures for brakes or structural corrosion are serious. Check the full history, not just whether it currently has a valid MOT.
How do I check the full MOT failure history of a car? Through the DVSA's free MOT history checker at check.mot.gov.uk, or automatically on every listing using the Don't Buy A Lemon Chrome extension.