MOT corrosion failures and the 30cm rule

By Martin Harvey, MH Auto Services · Published 19 October 2026

The short answer

A car fails its MOT on rust when the corrosion is in the load-bearing structure that important parts are mounted to, or within 30cm of those mountings, and it has made a hole or left the metal soft. Rust elsewhere, or rust that's only on the surface, doesn't fail on its own. The fix for a structural failure is a welded repair in sound metal.

What the MOT calls prescribed areas

The MOT inspection manual, which testers work to, describes "prescribed areas" as the "load-bearing parts of the vehicle to which testable items are mounted". Testable items include the brakes, steering, suspension and seat belts.

It then widens that area. A prescribed area also covers "any load-bearing or supporting structure or supporting panelling within 30cm of the mounting location". That's where the 30cm rule comes from.

The manual gives an example: for a seat belt mounted on an inner sill, the tester must also consider the outer sill or its reinforcement, and any door pillar, floor panel or other structural part within 30cm. Even a plastic outer sill cover is included in that check.

When rust becomes a fail

There isn't a measurement for "too rusty". In a prescribed area, the manual tells the tester to reject the car if:

• "the corrosion has caused a hole in the metal"
• "the area does not feel firm when you press it with your finger and thumb"
• pressing with a finger, thumb or a corrosion assessment tool makes a hole

Fractures and poor repairs in prescribed areas are rejected too. Parts like suspension arms and steering rods are judged separately: they fail if corrosion has seriously reduced the thickness of the metal or caused a hole or split.

Where it usually shows up

The manual's own example points at the sills, and that matches what we see in the workshop. Sills sit low, collect water and road salt, and have seat belt mountings and jacking points nearby. Wheel arches and the chassis rails around suspension mountings are the other usual places. Rust on a door skin or a bolt-on wing looks bad, but it usually isn't in a prescribed area, so on its own it isn't a structural fail.

What happens next

If your car has failed on corrosion, the failure sheet will say where. GOV.UK says you can drive the car away if your current MOT is still valid and no dangerous problems were listed, but it must stay roadworthy. If a dangerous problem was listed, it needs repairing before you drive it.

The repair is a welded one. Patching over holes with filler won't pass in a prescribed area, because the tester is checking the strength of the structure. Once the metal has been put right, treating the underside with something like Lanoguard helps slow down the next round of rust.

We weld sills, chassis rails and wheel arches to MOT standard on cars and vans. Send us a photo of the failure sheet or the rusty area on WhatsApp to 07843 699463 and we'll tell you what's involved. See car welding and MOT welding.

Sources: MOT inspection manual, Appendix A; GOV.UK, Getting an MOT: after the test.