Defect library
MOT defects explained
Last updated
Quick answer
MOT certificates record defects in the tester’s own words, which are precise but rarely plain. This library translates the most common entries into what is actually wrong, how serious it is, what it typically costs to put right, and whether it is a job you could reasonably tackle yourself.
All defects
- ABS warning lamp indicates a fault
The anti-lock braking warning light is on, or it will not light at all when it should. Ordinary braking still works, but the system that stops the wheels locking under heavy braking is not available.
Typical repair: £90–£500
- Airbag warning lamp on, or airbag missing
The airbag or SRS warning lamp is telling you the restraint system has a fault, or an airbag that was fitted from the factory is missing. Either way it is a major defect and a failure, because the system cannot be relied on to fire in a crash.
Typical repair: £60–£500
- Anti-roll bar or drop link worn, insecure or detached
The bar that ties the two sides of an axle together has a worn link, bush or mounting. It is one of the cheapest suspension repairs, and the usual source of a rattle over broken surfaces.
Typical repair: £60–£180
- Ball joint worn
Play has developed in a steering or suspension ball joint. Affects handling and tyre wear, and worsens over time.
Typical repair: £120–£280
- Body has a sharp edge likely to cause injury
A panel, bumper or trim is damaged, rotted or hanging loose enough to cut somebody who brushes against it. The test is about injury and security rather than appearance, so a dent on its own is not a defect.
Typical repair: £80–£600
- Brake binding or imbalanced across an axle
Either a brake is not releasing properly, or the two brakes on one axle are pulling with noticeably different force. An imbalance of more than 30% across an axle is a failure, because it steers the car under braking.
Typical repair: £120–£420
- Brake calliper seized or leaking
A calliper is either stuck, so it drags or will not press the pad properly, or it is weeping hydraulic fluid. Both are failures, and a leaking calliper also contaminates the pad next to it.
Typical repair: £180–£450
- Brake disc below its minimum thickness
The disc is no longer thick enough. This is not the same as the scoring and pitting recorded as surface wear — the metal itself has gone past the limit stamped on it by the manufacturer, and no amount of new pads will fix that.
Typical repair: £200–£480
- Brake disc worn, pitted or scored
The brake discs have surface wear. Braking is still effective, but the discs and usually the pads should be inspected soon.
Typical repair: £180–£320
- Brake drum or shoe worn or contaminated
On a drum brake, the friction lining is down to its limit or has been soaked in oil or grease. Contaminated linings cannot grip properly, which is why the tester treats it the same as having no material left.
Typical repair: £130–£320
- Brake fluid contaminated or boiling point reduced
The brake fluid has absorbed water or picked up contamination, which lowers the temperature at which it boils. Boiled fluid turns to vapour, and vapour compresses, so the pedal goes soft exactly when you need it most.
Typical repair: £60–£130
- Brake fluid level below minimum
There is less fluid in the reservoir than the minimum mark allows. It is a straight failure, and because a sealed hydraulic system does not use fluid up, the level has fallen for a reason.
Typical repair: £40–£220
- Brake hose deteriorated, damaged or chafing
One of the flexible rubber hoses feeding a brake is perished, split, bulging or rubbing against something. Hoses are cheap, and a failed one takes the braking on that wheel with it.
Typical repair: £70–£180
- Brake pads wearing thin
Friction material on the pads is close to its limit. Straightforward to replace, and cheaper if done before the discs are damaged.
Typical repair: £90–£200
- Brake pipe corroded
A rigid brake pipe has rusted. Light surface corrosion is an advisory, but a pipe that is excessively corroded or has been weakened is a failure, because it carries the pressure that stops the car.
Typical repair: £90–£250
- Brake servo or vacuum system defective
The servo that multiplies your effort on the pedal is not doing its job, or the vacuum supply feeding it has failed. The brakes still work, but the pedal goes hard and you have to push far harder for the same stop.
Typical repair: £150–£650
- Brake system warning lamp on, or not working
A brake system tell-tale is reporting a fault, or is not lighting when it should. DVSA does not name the system, because the same defect wording covers the anti-lock, electronic braking and electronic parking brake lamps — so a code read is the only way to know what the car is actually complaining about.
Typical repair: £70–£550
- Braking efficiency below requirement
The vehicle did not achieve the required braking force on test. This is a failure item and needs diagnosis before further use.
Typical repair: £120–£450
- Catalytic converter missing or defective
A catalytic converter fitted by the manufacturer is missing, obviously modified or obviously broken. It has to be present and working if the car left the factory with one, and on most cars the gas readings will fail as well.
Typical repair: £200–£900
- Corrosion in a structural area
Rust has been found in or near a load-bearing area. Repair must be done properly by welding, not filled.
Typical repair: £250–£1,200
- Diesel emitting visible smoke
Smoke was visible from the tailpipe during the test. On any diesel built with a particulate filter, smoke of any colour at all is a failure regardless of what the opacity meter reads.
Typical repair: £150–£1,200
- Diesel particulate filter removed or tampered with
The filter that traps soot has been taken out, hollowed out or cut open, or the tester has found evidence that it has been interfered with. It is a straight failure, and using the car on the road in that state is a separate offence.
Typical repair: £450–£2,000
- Direction indicator not working
One of the indicators is not operating, or is flashing at the wrong rate. Usually a bulb or a corroded holder, and one of the cheapest failures to put right.
Typical repair: £5–£60
- Door or boot will not close securely
A door, boot lid or tailgate will not latch shut, will not open from the handle, or its hinge or catch has worn. A front door that could fly open while you drive is the version of this that gets graded dangerous.
Typical repair: £70–£400
- Driver's seat insecure or will not adjust
The seat moves on its mountings, its runners no longer lock, or the backrest will not stay upright. A seat that shifts under braking is graded dangerous, which makes this one of the few interior items that can stop you driving the car away.
Typical repair: £80–£400
- Driver's view of the road obstructed
Something fitted to or hanging in front of the windscreen is blocking the driver's view — a dashcam, a satnav mount, a sticker or a permit. The size allowed directly in front of the driver is much smaller than most people expect.
Typical repair: £0–£60
- Driveshaft CV joint gaiter split or missing
The rubber boot sealing a driveshaft's constant velocity joint has split or gone. The joint underneath is usually still sound when the tester finds it, which is the whole point — a new gaiter costs a fraction of the shaft it was protecting.
Typical repair: £120–£320
- Emissions above the permitted limit
The vehicle exceeded its emissions threshold. Causes range from a due service to a failing catalyst or sensor.
Typical repair: £80–£900
- Engine management light on, or not working
The engine malfunction indicator lamp — the orange engine outline most people call the engine management light — is lit. Since May 2018 that is a major defect on its own, whatever the car drives like and whatever the gas readings say.
Typical repair: £60–£850
- Exhaust leaking or insecure
Gases are escaping before the tailpipe, or a section is loose. Can be a clamp and a joint, or a full section.
Typical repair: £60–£350
- Exhaust mounting deteriorated or missing
A rubber hanger or bracket holding the exhaust up has perished, broken or gone altogether. On its own that is not automatically a failure — the tester judges whether the system as a whole is still secure.
Typical repair: £20–£120
- Fuel filler cap missing or fuel system leaking
Fuel is escaping from the tank, a pipe or a hose, or the filler cap is missing or no longer seals. A missing cap counts the same as a leak, so it is worth checking yours is there and tightens properly before the test.
Typical repair: £15–£500
- Headlamp aim incorrect
The beam pattern is outside the permitted range, which can dazzle other drivers. Usually a quick adjustment.
Typical repair: £20–£60
- Headlamp not working
A headlamp did not illuminate on test. Usually a bulb, but can be wiring, a connector or a failed control module.
Typical repair: £20–£120
- Horn not working, or of the wrong type
The horn did not sound, was too quiet to be useful, or is a type the rules do not allow. Anything that alternates between notes — an air horn, a musical horn, a two-tone — fails even when it works perfectly.
Typical repair: £20–£120
- Lamp not working
A side, brake, indicator, fog or registration plate lamp failed to operate. Typically a bulb or a corroded holder.
Typical repair: £10–£80
- Lamp of the wrong colour, position or intensity
The lamp works perfectly well — it is simply the wrong colour, sitting in the wrong place, or throwing out more or less light than the rules allow. Aftermarket LED bulbs in headlamps built for filament bulbs, and tinted rear lenses, are the two causes a tester meets most often.
Typical repair: £10–£250
- Mileage discrepancy
A recorded odometer reading is lower than an earlier one. Check the original certificates, service invoices and the instrument history before drawing a conclusion — a fallen reading can be a data-entry error or a replaced instrument cluster, not necessarily alteration.
Typical repair: Verification is usually free
- Mirror missing, damaged or insecure
A mirror the vehicle is required to have is missing, broken, or too loose to give a steady view behind. Not every mirror on the car is a test item, which is why some smashed mirrors still pass.
Typical repair: £25–£250
- Parking brake efficiency too low
The handbrake did not produce enough braking force on the rollers. Most cars must reach 16% efficiency, and a parking brake that falls short will not reliably hold the vehicle on a hill.
Typical repair: £120–£420
- Parking brake lever or cable travel excessive
The handbrake pulls up too far, will not hold at the ratchet, or its cable has seized. It usually means the brake is out of adjustment rather than worn out, but it is still a failure.
Typical repair: £90–£320
- Play in the steering
Excess movement has been found in the steering system. Needs diagnosis to identify which joint or rack is responsible.
Typical repair: £150–£600
- Power steering fluid leaking or below minimum
Hydraulic fluid is escaping from the steering system, or the reservoir has dropped below its minimum mark. A weep is usually recorded rather than failed, but it is the warning that comes before the pump or rack gives up.
Typical repair: £90–£400
- Power steering warning lamp on, or assistance not working
The steering system is reporting its own fault, or the assistance has dropped out. The car remains steerable but takes far more effort, particularly when parking, and an illuminated steering warning lamp is a failure in itself.
Typical repair: £70–£850
- Registration plate missing, insecure or illegible
A number plate is missing, hanging loose, or no longer readable — faded, cracked, dirty, obscured or made up with the wrong font or spacing. It fails the test and is a separate offence on the road.
Typical repair: £20–£60 per plate
- Road spring broken or fractured
A suspension spring has cracked or snapped, usually near its bottom coil. The corner it sits on drops slightly and the car no longer holds its geometry, so it is a failure rather than something to watch.
Typical repair: £140–£320
- Road spring seat or suspension turret corroded
The steel the spring or strut pushes against has rusted through or thinned badly. Because it sits inside a prescribed area, the repair has to be cut out and welded rather than patched or filled.
Typical repair: £250–£800
- Road wheel damaged, distorted or cracked
A wheel is buckled, cracked or badly deformed. Kerb scuffing on the face is cosmetic and not a defect; damage that reaches the rim flange, the spokes or the bead seat is another matter.
Typical repair: £60–£350 per wheel
- Seat belt anchorage corroded or weakened
The metal the seat belt bolts into has rusted or been weakened within the 30cm prescribed area around the anchorage. It has to be cut out and welded to original strength — filler and plated patches are not accepted.
Typical repair: £250–£800
- Seat belt damaged or not working properly
A belt is cut, frayed, stretched, or its buckle or retractor no longer works as it should. Every seat belt fitted to the car is tested, not just the front pair, and there is no advisory grading for one that has failed.
Typical repair: £60–£300
- Shock absorber defective
A damper is leaking or no longer controlling the spring. Replaced in pairs across an axle to keep handling balanced.
Typical repair: £180–£400
- Steering rack gaiter split, damaged or missing
The rubber boot sealing the end of the steering rack has split. On its own it is a cheap part, but once grit and water are inside the rack the damage is permanent, so this is a failure worth acting on quickly.
Typical repair: £80–£200
- Steering rack mounting insecure or deteriorated
The rack itself is moving on the bulkhead or subframe it bolts to, usually because a mounting bush has collapsed or a bolt has come loose. The steering feels imprecise even though every joint is sound.
Typical repair: £120–£450
- Steering wheel or column insecure
The steering wheel or the column behind it is not properly secure, or a coupling in the column is damaged. It is one of the few faults where the link between your hands and the road wheels is itself in question.
Typical repair: £90–£450
- Stop lamp not working
A brake light is out. It is a straightforward fix, but it removes the main warning you give a following driver, so it is treated as a failure rather than an advisory.
Typical repair: £5–£70
- Suspension arm bush worn
A rubber suspension bush is deteriorating. It can cause knocking, uneven tyre wear and less precise steering as it worsens.
Typical repair: £160–£300
- Suspension arm damaged, distorted or corroded
A suspension arm is bent, cracked or rusted beyond what the tester will accept. Arms set where the wheel sits, so a distorted one throws the alignment out and cannot be straightened — it is replaced.
Typical repair: £150–£340
- Suspension joint dust cover missing or split
The small rubber cap sealing a suspension ball joint has split or gone altogether. The joint beneath it is usually still tight — the cover is what keeps it that way, which is why a missing one is a failure in its own right.
Typical repair: £90–£280
- Suspension mounting corroded or insecure
A suspension component is loose on its mounting, or the metal around the mounting has corroded. Rust within 30cm of a suspension mounting point falls in a prescribed area, where DVSA sets the standard the repair has to meet.
Typical repair: £120–£700
- Track rod end has excessive play
One of the joints connecting the steering rack to the wheel has worn loose. It makes the steering vague and eats the inner edge of a front tyre, and it is among the cheapest steering repairs to put right.
Typical repair: £90–£200
- Tyre damaged or deteriorated
The tyre has a cut, bulge or perishing sidewall. This cannot be repaired — the tyre needs replacing.
Typical repair: £70–£160 per tyre
- Tyre fouling part of the vehicle
A tyre is touching the bodywork, an arch liner, a spring or a brake hose as the wheel moves. Usually the wrong wheel or tyre size, or a suspension part that has shifted out of position.
Typical repair: £40–£300
- Tyre load index or speed rating below requirement
The tyre is not rated to carry the weight or run at the speed the manufacturer specifies for this vehicle. It usually follows a cheap replacement, or car tyres fitted to a van.
Typical repair: £80–£320 per axle
- Tyre ply or cord exposed, or the casing separating
The tyre's internal structure has failed — cords or ply are showing through the rubber, or the casing has begun to come apart. This is graded dangerous outright, and the tyre cannot be repaired.
Typical repair: £70–£160 per tyre
- Tyre pressure monitoring system inoperative
The tyre pressure monitoring system is not working, or is reporting a fault. On vehicles first used from 1 January 2012 that is a failure in its own right, whatever the actual pressures are.
Typical repair: £60–£150 per wheel
- Tyre valve damaged or tyre not correctly seated
The valve is damaged, missing or leaking, or the tyre has not sealed properly onto the rim. Cheap to put right, and it is the only thing holding the air in.
Typical repair: £10–£90 per wheel
- Tyre worn close to the legal limit
There is little usable tread left. Replace before it reaches the 1.6mm legal minimum across the central three-quarters of the tyre.
Typical repair: £70–£140 per tyre
- Tyres of different sizes or construction on the same axle
The two tyres on one axle are not the same size, or mix radial and cross-ply construction. Different makes and tread patterns side by side are legal in the UK; different sizes and structures are not.
Typical repair: £70–£280 per axle
- Wheel bearing has excessive play or is rough
A wheel bearing has worn enough to let the hub move, or has gone rough inside. It gives a droning noise that rises with speed and changes when you steer, and it does not recover — bearings only get worse.
Typical repair: £140–£320
- Wheel nut, bolt or stud missing, loose or damaged
One of the fixings holding a wheel to the hub is missing, slack or damaged, so the rest are carrying a load they were never meant to take alone. Do not drive on it.
Typical repair: £15–£180
- Window glass damaged or too heavily tinted
Glass other than the windscreen is cracked, shattered or tinted too dark. What counts is whether it affects the driver's view of the road, and how much light the front glass lets through.
Typical repair: £70–£350
- Windscreen damaged in the swept area
A chip or crack sits within the driver's field of view. Small chips can often be resin-repaired rather than replaced.
Typical repair: £70–£350
- Wiper or washer not working properly
The wipers or washers cannot clear the screen adequately. Usually blades, a blocked jet or a failed pump.
Typical repair: £15–£120
How defects are graded
The same component can be recorded at different severities depending on how bad the fault is. A worn ball joint might be an advisory one year and a major defect the next without anything dramatic happening in between.
Dangerous and major defects fail the test. Minor defects and advisories both pass, but both are permanently recorded against the vehicle. The full explanation is in our guide to dangerous, major and minor defects.
Common questions
- What is the most common MOT failure?
- Lighting and signalling faults are consistently the largest single category, followed by suspension, brakes and tyres. Most are cheap and quick to put right, which is why a pre-test walk round the car is worth the ten minutes it takes.
- What is the difference between a defect and an advisory?
- A defect graded dangerous or major fails the test. An advisory is the tester telling you something is wearing but still within the limit, so the vehicle passes with the note recorded. Minor defects also pass while being recorded against the vehicle.
- Do advisories have to be fixed?
- Not legally, and the vehicle passes with them. But an advisory is a prediction: an item advised as nearing the limit this year is the item that fails next year. Fixing it before the next test is usually cheaper than the retest that follows a failure.
- Can I drive my car if it fails its MOT?
- Only in narrow circumstances. If the previous MOT has not yet expired and no dangerous defect was recorded, you may drive it. With a dangerous defect, or once the old certificate has expired, you may only drive to a pre-arranged repair or test appointment.
- How much does it cost to fix a typical MOT failure?
- The most common failures are inexpensive. A bulb is a few pounds, wipers or a washer jet under twenty, and brake pads around a hundred and fifty fitted. The costly outliers are catalytic converters, diesel particulate filters and structural corrosion needing welded repair.
- Where does this information come from?
- The grading rules come from the DVSA MOT inspection manual, which sets out how each item is assessed and at what point it becomes a failure. The plain-English explanations and repair cost ranges are ours, written to make the tester's wording legible.
See what a specific car has been advised on
Every defect and advisory ever recorded, decoded.