MOT Rules For Modified Cars: Exhausts, Coilovers, Wheels & LEDs

If you daily a slammed or tuned motor, keeping on top of MOT rules for modified cars is the difference between a clean pass and a tester shaking their head while you ring round for a trailer.

What MOT rules for modified cars actually care about

Contrary to the pub chat, the MOT is not there to kill the scene. Testers do not care how wild your build is, they care about safety, noise and emissions. The key points for most modified cars are:

• Exhaust noise and leaks
• Cats, DPFs and emissions
• Ride height and coilover setup
• Aftermarket wheels and tyre fitment
• Lighting and LED conversions

As long as the car is safe, not obviously illegal and not taking the mick with noise or smoke, a switched on tester will usually be fine.

Exhaust noise, decats and emissions

Exhausts are where a lot of people get caught out. A performance system is fine as long as:

• It is not leaking
• It is secure with proper hangers
• It is not "excessively loud" compared to a standard car

"Excessively loud" is subjective, but if your car is antisocial at light throttle or needs ear defenders in the cabin, expect questions. Pop and bang maps that fire on command can also wind testers up, so stick it in the quietest mode you have.

On emissions, the big one is missing cats and DPFs. If the car left the factory with a cat or DPF, it must still be there and look genuine. A straight pipe in a fake cat shell is asking for a fail. Petrols have to hit the gas test numbers and diesels are checked for visible smoke and DPF presence.

If you are running a decat for track use, the safest play is to keep a catted section ready to bolt back in for MOT time. Sort any boost leaks and misfires before the test too, as both can ruin your emissions numbers.

Coilovers, ride height and clearance

Coilovers themselves are not a problem. MOT rules for modified cars only really care about how they are fitted and how the car behaves. You need to tick these boxes:

• Springs seated properly at full droop – no loose springs
• No fouling on tyres, bodywork or brake lines at full lock
• Dampers not leaking and securely mounted
• Enough ground clearance that nothing critical is scraping

If your daily is sat on the floor, wind it up a touch for the test. Give yourself enough room so the exhaust, fuel lines, sump and chassis rails are not polishing the MOT station floor. A quick four-corner tweak and tracking check before the test can also sort out any weird handling that might worry a tester on the brake rollers.

Aftermarket wheels, stretch and poke

Aftermarket wheels are fine as long as the basics are right. The tester is looking for:

• No tyre contact with arches, liners or suspension
• Correct load and speed rating on the tyres
• No cracks, chunks missing or exposed cords
• Wheel nuts or bolts long enough and properly seated

A bit of stretch and poke is not an automatic fail, but if the bead looks sketchy or the tread sits outside the arch throwing crud everywhere, you are relying on tester mood. For cars on aggressive fitment, it is worth having a "MOT set" of wheels and tyres you can swap on for the day.

LED conversions, light bars and MOT

Lighting is an area where testers have got a lot stricter. Factory LED or xenon setups are fine, but cheap LED bulbs thrown in halogen housings are a grey area. Testers are checking for:

• Correct colour and aim – no blinding oncoming traffic
• Working dip, main, indicators and brake lights
• No flicker, loose units or insecure wiring

Many will fail obvious plug-in LED headlight bulbs that scatter light everywhere. If you want hassle free MOTs, keep proper halogen bulbs in for the test or invest in a full, type-approved headlamp upgrade rather than eBay specials.

MOT rules for modified cars FAQs