The short answer
Check every bulb, the wipers, the screenwash, the tyres, the horn and the number plate. A large share of MOT failures are for things that take ten minutes to fix at home.
- Every light: headlights on both beams, sidelights, indicators, hazards, brake lights including the high-level one, reversing light, fog lights, number plate lights.
- Wipers and washers: blades not split, washers aiming at the screen, reservoir full.
- Screen: any chip in the driver's line of sight larger than 10mm is a failure.
- Tyres: tread and condition, including the sidewalls.
- Horn, seatbelts, mirrors and number plate legibility.
- Warning lights: an illuminated airbag, ABS or engine management light is a failure in its own right.
What will ruin it
Do not leave it until the day. Book a week before the expiry date, which gives you time to fix anything found without the car becoming untaxable and uninsurable overnight.
Why it works
The commonest failure categories are lighting, suspension, brakes and tyres, and lighting alone accounts for a very large share. It is the one category any owner can check completely in a driveway.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.