How the MOT Test Works
The MOT (Ministry of Transport) test is a legal requirement for most vehicles over three years old in the UK. It checks that your vehicle meets road safety and environmental standards. From May 2018, the test categorises defects as:
- Dangerous: The vehicle must not be driven until repaired
- Major: A fail that requires repair before the vehicle can be driven on the road
- Minor (Advisory): Not a fail, but should be addressed soon
An MOT checks over 30 components. The most common reasons for failure come up again and again, and most are avoidable.
The Most Common MOT Failure Reasons
Lighting and Signalling
Faulty lights are one of the top causes of MOT failure. This includes blown bulbs, cracked lenses, and lights that are incorrectly aimed.
Check before your test:
- All headlights, including full beam
- Brake lights (ask someone to watch while you press the pedal)
- Indicator lights, front and rear
- Fog lights and reversing lights
- Number plate light
Replacing a bulb costs a few pounds and takes minutes. There is no excuse for failing on a light.
Tyres
Tyre failures include insufficient tread depth (below 1.6mm), damage to the sidewall or tread, incorrect tyre type mixing, or a tyre that is the wrong size for the vehicle. Check tread depth across all four tyres and inspect for damage and bulges before your MOT.
Brakes
The MOT includes a brake efficiency test using a rolling road. It also involves a visual check of pad depth, disc condition, and brake fluid level. Worn pads, scored discs, uneven brake performance, or a contaminated brake fluid reservoir can all cause failures.
Suspension and Steering
Worn suspension components including shock absorbers, ball joints, wishbones, and anti-roll bar links are a common failure point. Inspectors check for excessive play, leaking dampers, and corrosion.
A simple check: push down hard on each corner of the car. It should bounce once and return to its position. Excessive bounce suggests a worn shock absorber.
Windscreen and Wipers
Any crack or chip in the driver's line of vision (a 10mm zone directly in front of the driver) is an automatic fail. A chip or crack larger than 40mm in the A-pillar zone (the area swept by the wipers, excluding the 10mm zone) is also a fail.
Wiper blades that smear or leave clear patches in the swept area will also fail.
Emissions
Diesel vehicles are tested for particulate emissions and can fail if the exhaust smoke is excessive. Petrol vehicles are tested for CO, CO2, and hydrocarbon levels.
Common causes of emissions failures:
- A blocked or failing diesel particulate filter (DPF)
- Worn spark plugs on petrol engines
- A faulty oxygen sensor
- Old, degraded engine oil
Seatbelts
All seatbelts must work correctly. Belts that do not retract, latch, or release properly will fail. Check all seatbelts in the car, not just the driver's.
Horn
The horn must work. It is a simple check that catches some drivers off-guard. Press it before you go.
Number Plates
Number plates must be clean, undamaged, and show the correct font and spacing. Modified or novelty plates with incorrect spacing or unusual fonts may fail. The registration must also be correctly lit at night.
How to Reduce Your Risk of MOT Failure
Run through this checklist in the week before your MOT:
- Check all lights work correctly
- Inspect tyres for tread depth and damage
- Check washer fluid and wiper operation
- Test your horn
- Make sure your number plate is clean and legible
- Check for any warning lights on the dashboard
- Listen for unusual noises from the brakes or suspension
- Check the windscreen for chips or cracks
If your car has a service due, getting it serviced shortly before the MOT is worthwhile. Fresh oil, a new air filter, and fresh spark plugs on a petrol car can all help with emissions.
What Happens If Your Car Fails?
If your car fails, you have options. If the failure is corrected at the same testing station within ten working days, you may qualify for a partial retest at a reduced cost rather than paying for a full retest. The rules vary slightly depending on the garage.
Driving a car that has failed on a dangerous defect is illegal. For major defects, you can drive the car only to a pre-booked repair appointment.
