Common MOT failures on modified cars and how to avoid them

Modified cars don't fail their MOT for mysterious reasons. They fail for a small, predictable set of reasons that a bit of checking beforehand can usually catch. Here's what's actually worth looking at before you book it in.

1. Check your tyres and wheel fitment. Stretched tyres, uneven wear from bad geometry, and wheels that foul the arch or bodywork under full lock or compression are all common, checkable issues.
2. Look at your ride height. Some testers will flag a car that sits noticeably lower than standard if it affects headlight aim or ground clearance around the exhaust and sump. Check yours sits sensibly before the test, not just that it looks right to you.
3. Check your exhaust and emissions setup. A decat or sports cat can be a genuine MOT risk on a car that needs to pass an emissions test, since the catalytic converter is doing real work that a straight pipe doesn't replace.
4. Check every light actually works and is the correct colour. Aftermarket bulbs, tinted light covers, and non-standard LED conversions are a common, easy to overlook failure point.
5. Check your number plate is legible and correctly formatted. Coloured plate surrounds, non-standard fonts, and anything obscuring the characters are a straightforward fail.

Tyres and wheel fitment

Stretched tyres look the part but can fail on tread contact width or bead seating depending on how extreme the stretch is. Spacers that push the wheel out far enough to catch the arch under compression are a genuine, physical problem, not just a cosmetic one. If you've had aftermarket wheels or spacers fitted, a proper geometry check afterward catches most of the wear-related issues before they become an MOT problem.

Ride height and suspension

There's no single legal minimum ride height in the same way there's a legal maximum for some modifications, but a tester can and will fail a car if the modification affects headlight aim, ground clearance, or causes components to foul under normal movement. A geometry check after lowering a car isn't just about tyre wear, it's part of keeping the car within what a tester will pass.

Exhaust and emissions

Post-2006 petrol cars in particular need to pass an emissions test that a genuine catalytic converter is designed around. Removing or replacing it with a decat pipe is a real, common cause of an emissions failure, not an urban myth. If emissions matter to you passing, keep a genuine cat in the system, even if it's a sports cat rather than the stock one.

Lighting and visibility

Any light that's supposed to be white and shows anything other than white, any bulb that doesn't match its housing's intended type, and any cover that changes the colour or significantly reduces brightness are all straightforward, checkable failure points. This is one of the cheapest things to get right before a test and one of the most common things people overlook.

Keep a maintenance log so nothing surprises you

Setting your MOT date on your Car Scene garage profile means you get a reminder before it's due, which gives you time to actually check these things rather than discovering a problem at the test centre. It's a small habit that turns MOT day from a gamble into a formality.