The Most Common MOT Failures and How to Avoid Them

Most MOT failures are not dramatic. They are not seized engines or collapsed subframes. They are bulbs, worn tyres, torn wiper blades and suspension components that have been quietly deteriorating for two years. DVSA's published MOT statistics have consistently shown that around a third of cars fail at the first attempt, and a very large share of those failures involve items the owner could have found in fifteen minutes with no tools.

This guide works through the failure categories in roughly the order they appear in the national figures, explains what actually goes wrong in each one, and sets out the specific check that prevents it.

How MOT failures are categorised

Every defect recorded at an MOT is filed against a section of the DVSA testing standard. The sections that generate the most defects, in broad order of volume according to DVSA's published statistics, are:

1. Lamps, reflectors and electrical equipment
2. Suspension
3. Brakes
4. Tyres
5. Driver's view of the road
6. Body, structure and general items
7. Seatbelts and supplementary restraint systems
8. Exhaust, fuel and emissions
9. Steering
10. Registration plates and vehicle identification

The ranking shifts a little from year to year and varies by vehicle age, but the top five have been stable for a long time. Note that these are counts of recorded defects rather than counts of failed vehicles — a single car can contribute several — and that each category contains both minor items and serious ones.

Since May 2018 every defect is graded as dangerous, major, minor or advisory. Dangerous and major both fail the test; minor and advisory do not, though both are permanently recorded on the vehicle's public record.

Lighting and signalling: the biggest category by a distance

Lamps generate more recorded defects than any other section, and they are the cheapest to prevent.

What actually fails:

• Blown bulbs. Brake lights and the high-level brake lamp are the worst offenders, because you cannot see them from the driver's seat.
• Headlamp aim. Beams that are too high, too low or pointing the wrong way laterally. Aim drifts after a bulb change, a front-end knock, a suspension repair or even a change in ride height.
• Damaged or insecure lenses. Cracks that let water in, lenses held on with tape, and units hanging loose in the wing.
• Discoloured or fogged headlamps where the plastic has oxidised enough to affect the beam pattern.
• Wrong colour or type, including aftermarket LED bulbs fitted into a housing designed for halogen, which typically produces an unacceptable beam pattern.
• Rear fog lamp not working — a single working one is required on vehicles first used from April 1980.

The check

Park facing a wall or garage door at night, switch on sidelights, dipped beam, main beam, fog lamps and hazards in turn, and walk around the car. Have someone press the brake pedal while you stand behind, and check the high-level lamp. Then reverse gently towards the wall to confirm the reversing lamps work.

If your dipped beam pattern on the wall is obviously uneven between left and right, or if oncoming drivers have started flashing you, get the aim set. Headlamp aim adjustment is a quick job and cheap compared to a failure. The defect entries for headlamp not working and headlamp aim explain how each is graded.

Time required: five minutes. Cost of prevention: the price of a bulb.

Suspension: the biggest category you cannot easily check

Suspension is consistently near the top of the failure statistics and is much harder for an owner to self-assess, because most defects are found by levering a component to measure play.

What actually fails:

• Worn ball joints and track rod ends, where movement has developed in the joint.
• Perished or split bushes on suspension arms and anti-roll bar links.
• Broken coil springs, extremely common on cars that spend winters on salted roads, and often producing no symptom at all until the tester spots the fracture.
• Leaking or ineffective shock absorbers. Visible fluid on the damper body is a defect.
• Corroded or damaged suspension components and mounting points.

The check

You cannot measure play without lifting the car, but you can pick up strong hints:

• Listen. Knocking over speed bumps, clonking on turn-in, or a rattle from the front over rough surfaces almost always means a worn link, bush or joint.
• Look at the tyres. Uneven wear on the inner or outer shoulder points at alignment, which in turn often points at a worn suspension component.
• Bounce test. Push down hard on each corner and release. The car should settle in roughly one rebound. Continued bouncing suggests tired dampers.
• Look at the springs. With the wheels on full lock you can often see the coils on the front. A broken spring usually snaps near the bottom coil and is visible once you know to look.

Anti-roll bar drop links are the classic cheap fix here — often under £100 fitted for a pair — while a failed spring or a worn suspension arm bush costs more. Getting a garage to check the front end before the test is worthwhile on any car past about eight years old.

Time required: ten minutes of looking and listening. Best prevented by a pre-MOT inspection.

Brakes: measured, not judged

Brake defects split into two groups: the condition of components, and the measured performance on the roller brake tester.

What actually fails:

• Friction material worn below 1.5mm, or discs that are badly scored, cracked or lipped.
• Service brake efficiency below 50% for most cars, or parking brake efficiency below 16%.
• Imbalance across an axle of more than 30%, usually caused by a seized calliper or slider on one side.
• Corroded brake pipes and perished flexible hoses.
• Handbrakes that no longer hold, particularly on cars with rear drums or rear-calliper parking brake levers.
• ABS or stability control warning lamps illuminated.

The check

• Look through the wheel spokes at the front disc. A pronounced lip at the outer edge and a thin remaining pad both mean it is time.
• Note whether the car pulls to one side when you brake firmly. That is imbalance you can feel.
• Check the brake fluid level in the reservoir.
• Use the handbrake regularly rather than always parking in gear; on cars with self-adjusting rear drums this genuinely maintains adjustment.
• If you are having brake work done, do it at least a week before the test so the new material beds in properly. Fresh pads tested the same day frequently under-read.

The full mechanics of the measurement are covered in the guide to brake testing in the MOT.

Time required: five minutes. Cost of prevention: nothing, but the repairs themselves are not cheap.

Tyres: the most preventable serious failure

Tyres are the category where the rules are entirely objective and the owner has complete control.

What actually fails:

• Tread below 1.6mm across the central three-quarters of the width, anywhere around the circumference.
• Cuts, bulges and exposed cord, usually from kerb strikes and potholes.
• Incorrect size, speed rating or mismatched types across an axle.
• Perished sidewalls on low-mileage cars.
• Inoperative tyre pressure monitoring on vehicles first used from January 2012.

The check

Buy a tread depth gauge. Measure each tyre at the inner, centre and outer positions, and at three or four points around the wheel, then take the worst reading. Turn the steering to full lock to inspect the inner shoulders of the front tyres, which is where alignment-related wear hides and where you cannot see it otherwise.

Set the pressures to the figures on the plate in the driver's door shut or inside the fuel flap, and inspect the sidewalls for bulges while you are there. Full detail on the measurement rules is in the guide to tyre tread depth law.

Time required: ten minutes. Cost of prevention: the price of a gauge.

The driver's view of the road

A large and very avoidable category.

What actually fails:

• Windscreen damage. In zone A — the 290mm-wide strip directly in front of the driver within the wiper sweep — damage larger than 10mm fails. Elsewhere in the swept area the limit is 40mm.
• Wiper blades that smear, judder or are torn.
• Washers that do not deliver enough fluid to clear the screen. An empty bottle is a straight fail.
• Mirrors missing, cracked or insecure.
• Obstructions in the swept area, including large dashcam or satnav mounts, air fresheners and stickers.

The check

Top up the washer bottle and test both front and rear jets. Run the wipers on a wet screen and watch for streaks. Measure any chip in front of the driver — a 10mm chip is smaller than most people picture. Windscreen chips can often be repaired for far less than a replacement if you deal with them before they spread; the windscreen damage entry sets out the zones in more detail.

Time required: three minutes.

Body, structure, seatbelts and identification

These categories generate fewer defects but include some of the most expensive ones.

• Structural corrosion within 30cm of a seatbelt anchorage, suspension mount, steering component or brake mounting is a failure requiring proper welded repair. On a car past a certain age this can be the failure that ends its economic life.
• Sharp edges from damage or rust.
• Insecure doors, bonnet or boot.
• Seatbelts that are frayed, cut, fail to retract or do not lock under sharp pull. Every belt fitted is checked.
• Airbag warning lamp illuminated, or an airbag removed rather than repaired.
• Number plates that are cracked, faded, illegible, incorrectly spaced or in a non-standard font. Rearranged spacing on a personalised plate is a straightforward fail.
• VIN missing, illegible, or more than one present.

The check

Tug each seatbelt sharply — it should lock. Pull each one fully out and look along its length for fraying at the edges. Check that every belt retracts cleanly. Press the horn. Turn the ignition to position two and confirm the airbag, ABS and engine management lamps illuminate and then go out. Clean the number plates and look at them from a few metres back.

Time required: five minutes.

Exhaust and emissions

Fewer defects than the categories above, but often more expensive when they occur.

• Exhaust leaks, insecure mountings and systems that no longer silence the vehicle effectively.
• Emissions above the limits for the vehicle's fuel type and date of first use.
• Any visible smoke of any colour from a diesel with a factory-fitted particulate filter is a major defect.
• Evidence of DPF removal or tampering, which means the tester must refuse the test.

The check

Start the car from cold in a quiet place and listen for a blowing noise that changes with engine speed. Look for black soot around joints. Before an emissions test, get the engine fully up to temperature — a good half-hour run, including some sustained higher-rev motorway driving on a diesel, gives the DPF a chance to regenerate and clears carbon from the system. Make sure the engine oil is not overdue for a change, because degraded oil affects diesel smoke readings.

A pre-MOT checklist that takes fifteen minutes

None of this requires tools beyond a tread depth gauge, and together these items account for a substantial slice of first-time failures.

What to do if it fails anyway

Some failures cannot be prevented by a driveway inspection, and a failure is not the disaster it feels like.

You will be given a refusal of an MOT test certificate, form VT30, listing every defect with its grade. Read it before agreeing to anything. You are entitled to take the vehicle elsewhere for repair, and no test centre can require you to have the work done there.

Whether you can drive the car away depends on the grade and on your existing certificate. If the failure is major or minor and your previous certificate has not yet expired, that certificate remains valid and you may keep driving until it runs out. If the vehicle failed with a dangerous defect, you must not drive it away at all. If your previous certificate has already expired, you may only drive to a pre-arranged repair or a pre-booked retest.

Retest fees are more generous than most people assume. Leave the car at the centre for repair and the retest is free. Take it away and bring it back before the end of the next working day and the retest is free for a defined list of straightforward items, which includes lamps, wipers, mirrors, registration plates, seatbelt condition and the fuel filler cap. Return within ten working days for a partial retest and a reduced fee may apply. After that you pay for a full test, which for a class 4 vehicle is capped at £54.85.

Use last year's certificate as the plan

The best single predictor of what a car will fail on is what a tester already told you about it. Advisories are not decorative: they are a written note that a component is deteriorating and has roughly twelve months left. Tyres marked as close to the limit, brake pipes noted as corroded, and joints with slight play recorded against them frequently reappear the following year as major defects.

Pull up your car's previous results using the MOT history check, read the advisories from the last two tests, and deal with them before you book. If you are unsure when the test is due, the MOT due date lookup answers it from the registration. For the full scope of the inspection, see the guide to what is checked on an MOT.