Common MOT failures and the parts that fix them

Before the test: the free five-minute check

• Walk round with someone pressing the pedal and switches: headlights (dipped and main beam), sidelights, brake lights, indicators, hazards, fog light and number plate lights.
• Tyres: at least 1.6mm of tread across the middle three-quarters of the tyre, all the way round, with no cuts or bulges.
• Wipers and washers: the blades clear the screen and the washer bottle is full.
• Windscreen: no chips or cracks in front of the driver, and no stickers or sat nav mounts in the way.
• Warning lights on the dashboard: the engine management, ABS, airbag and power steering lights should come on with the ignition and then go out.
• Horn, seat belts, mirrors and number plates: all there, working and readable.

Lights and electrics

Lighting faults are among the most common reasons for a fail, and often the cheapest to fix: a blown bulb, a cracked or cloudy lens, or a headlight aimed in the wrong place. A used headlight or rear light unit from a breaker is often far cheaper than new; match the side (nearside and offside), whether it's halogen, xenon or LED, and make sure it came from a UK (right-hand-drive) car so the beam dips the right way.

A dashboard warning light that stays on is a fail for the system it belongs to, such as ABS or airbags. Get the fault code read first: it may be a sensor or a plug rather than an expensive module. If it is a module, see buying a used ECU, as many need coding to your car.

Tyres and wheels

Tread below the 1.6mm legal limit, cuts, bulges and mismatched tyre sizes on the same axle all fail. Part-worn tyres must be sold with their tread depth and condition checked, so ask before you buy. A damaged alloy or a bent steel wheel can be swapped for a used one with the same size, offset and stud pattern: see wheel sizes and stud patterns.

Brakes

Testers check how well the brakes work on a rolling road, and look at the discs, pads, pipes and hoses. Worn pads and discs are usually best bought new, as they wear out and are cheap. Bigger parts such as calipers, ABS pumps, handbrake mechanisms and brake servos are good buys used: match the part number and the size of the disc your car has, as the same model often came with two sizes. A handbrake that doesn't hold the car is a common fail on older cars, often a seized caliper or a stretched cable.

Suspension and steering

Broken coil springs, worn ball joints and bushes, leaking shock absorbers and play in the steering are all common fails, especially on older cars on rough roads. Springs and shock absorbers are usually replaced in pairs on the same axle so the car sits and handles evenly. Wishbones (control arms), hubs, steering racks and anti-roll bar links are all widely available used: check for split rubber bushes and play in the joints before you buy, and match the part number, as sports and standard suspension often differ.

Windscreen, wipers and mirrors

A chip or crack in front of the driver fails if it's bigger than 10mm, and anywhere else the wipers clear if it's bigger than 40mm. Small chips can often be repaired. A replacement windscreen is usually bought new and fitted by a glass specialist, but door mirrors, mirror glass and wiper motors and linkages are easy and cheap to find used. Match the mirror's features (heated, folding, indicator, camera) and the side.

Exhaust and emissions

A blowing or loose exhaust, a missing catalytic converter or a diesel particulate filter (DPF) that has been removed will fail, as will smoke above the limit for your engine. If a DPF has been taken out, the car fails even if it runs well. Back boxes and middle sections are cheap to replace; for the expensive parts see buying a used catalytic converter.

After a fail

A fail sheet lists dangerous and major defects (which fail) and minor defects and advisories (which pass, but are worth fixing soon). You can't drive a car with a dangerous defect until it's fixed. If you leave the car at the test centre for the repair, or bring it back within 10 working days, the retest is often free or at a reduced price, so it pays to fix it quickly. Many test centres are happy to fit a part you've bought yourself; ask first.

Find the part your car failed on

Search by your registration to see parts listed as fitting your car, from UK breakers and parts sellers. Have your fail sheet and old part handy, and check the part number with the seller before you buy. Search for a part, browse by make, or read how to find the right part.