Updated May 2026
Will my modified car pass its MOT?
Most UK modifications pass an MOT if they're fitted properly, comply with construction-and-use regulations, and don't affect safety or visibility. The risk areas are tints, lighting, suspension drops, decat exhausts and structural changes. MOT Checkup shows you any modification-related advisories from previous tests on a free MOT history check.
TL;DR
- Tints: front windscreen ≥75%, front sides ≥70% light
- Aftermarket lights must comply with DVSA approval markings (E-mark or BS)
- Lowered suspension fine if no rubbing and headlight aim still adjusts
- Decat exhausts on post-1992 petrol cars fail emissions
- Tell your insurer first — non-disclosure voids cover
Tinted windows — the percentages
| Window | Minimum light through |
|---|---|
| Front windscreen | ≥75% |
| Front side windows | ≥70% |
| Rear side windows | No MOT-relevant limit |
| Rear windscreen | No MOT-relevant limit |
Verify the current DVSA values before fitting film — standards evolve. The MOT tester checks visibility through these windows rather than measuring tint percentage directly, but a non-compliant tint will be flagged as restricting view of the road.
Wheels and suspension
- Aftermarket alloys — pass if correctly seated, tyres rated for the load and speed, no rubbing under steering lock or compression.
- Wheel spacers — must be secured with the correct length wheel studs and torqued. Loose or insufficient stud engagement fails.
- Lowering springs / coilovers — fine if the headlight beam can still be adjusted to the correct setting and the suspension travel is sufficient.
- Stretched tyres — fail if the tyre bead has visibly come away from the wheel rim or the tyre rating is too low for the wheel size.
Lighting modifications
- Aftermarket headlight units must carry an E-mark or BS approval. Unbranded eBay LED conversions almost always fail.
- Headlight bulbs must produce the correct beam pattern. Most LED retrofits in halogen housings fail because the beam pattern is wrong.
- Tinted lights — rear lamp tints fail if they reduce intensity below the minimum or change the colour.
- Underbody / interior LED strips — illegal as moving lighting and fail if visible from outside.
Exhaust modifications
- Decat pipes — fail emissions on petrol cars built after 1992 and on diesels with DPF requirements.
- Cat-back systems — pass if the new system isn't obviously louder than the OEM and is securely mounted.
- DPF removal — automatic fail on diesel cars originally fitted with one. The DVSA introduced a visual DPF check in 2014.
- Sports cats — pass if the car still meets the emissions standard for its year of registration.
Structural changes
Cuts to chassis members, removal of seats with belt mounts, welded-in cages near the prescribed corrosion areas, and any modification to crash structure can fail the MOT outright. Show-spec builds with cages, bucket seats and harness bars are normally inspected against original mounting integrity. See our corrosion in prescribed areas piece and methodology for sourcing.
Frequently asked questions
Do I have to declare modifications to the MOT tester?
Not formally — the MOT inspects the vehicle as presented. But you should tell your insurer about any modification before driving, including aftermarket wheels, suspension changes, exhausts and lighting. Failure to declare modifications to insurers can void cover even if the modifications themselves would have passed an MOT.
Will aftermarket alloy wheels fail the MOT?
Not in themselves, but they must fit safely (no rubbing on suspension or arches), the tyres must meet the load and speed ratings, and the wheel must be secured properly. Spacers are inspected for security; poorly fitted spacers fail. The total wheel/tyre combo also can't extend beyond the bodywork without arch extensions.
Are tinted windows legal for the MOT?
Front windscreen must let through at least 75% of light, and front side windows at least 70%. Rear side and rear windows have no MOT-relevant light-transmission limit. The MOT tester checks visibility through these windows; an aftermarket tint that fails the percentage test will fail the MOT. Police can also issue immediate prohibition notices for non-compliant front tints. Always verify current DVSA tint thresholds before applying film, as standards have evolved.
Will a louder exhaust fail?
An exhaust modification fails if it's noticeably louder than the original equipment specification, if it's been altered to bypass the catalytic converter on a vehicle that requires one, or if the system is insecurely mounted. Decat pipes on cars built after 1992 generally fail the MOT and the emissions test.
What about engine swaps?
Engine swaps are not specifically MOT-tested unless they affect emissions or visible structure. The car must still meet the emissions standard for its registration year. A modern engine in an older car must still pass the older car's emissions limits unless the V5C is updated to record the change. Major swaps usually require notification to the DVLA.