MOT defect file
Corrosion in a structural area
Last updated
Quick answer
Rust has been found in or near a load-bearing area. Repair must be done properly by welding, not filled.
- Typical cost
- £250–£1,200
- DIY viability
- Garage job
What you might notice
- Paint blistering or bubbling along the sills, wheel arches and the seams beneath the doors
- A dull, crunchy sound when a suspect area is tapped, instead of a solid ring
- Damp carpets or a musty smell in a footwell or the boot, which often means the floor has holed underneath
- Frequently invisible from above — the rot is on the underside, and the first many owners hear of it is the test result
What causes it
- Road salt and standing water, particularly where the drain holes in sills and box sections have silted up
- Stone chips and kerb scrapes breaking the paint film and letting water reach bare steel
- Poor previous repairs — filler, glass fibre or plated patches laid over rust that was never cut out
- Water leaks from a perished screen seal, a blocked sunroof drain or a failed boot seal, rotting the floor from the inside out
Advisory or failure?
Where the corrosion is matters far more than how much of it there is. Any area within 30cm of a seatbelt anchorage, a suspension or steering mounting, a brake component mounting or a body-to-chassis mounting is a prescribed area, and corrosion there that has weakened the structure — judged by the tester using finger and thumb pressure or a corrosion assessment tool — is a major defect and a failure. Where the weakening is severe enough to affect the vehicle's structural integrity, the grading is dangerous. Outside the prescribed areas the bar is lower: a holed floor or sill that does not weaken the structure may be recorded as a minor defect or an advisory, though corroded bodywork with sharp edges likely to cause injury is a major. The repair standard is strict — within a prescribed area the repair must be continuously seam welded and at least as strong as the original, and filler, glass fibre, pop-riveted plates and a coat of underseal over the top are not acceptable.
Can you fix it yourself?
Garage job
Welding in a prescribed area is structural work: it needs a MIG welder, the skill to lay a continuous seam in thin and often rotten metal, and the judgement to keep cutting back until you reach sound steel. It also has to be done clear of the fuel tank, fuel lines and brake pipes. Cosmetic rust on a wing or a door is a reasonable home project, and keeping sill drain holes clear and washing salt off the underside through winter genuinely slows the whole problem down — but the moment the rot is anywhere near a mounting point, it belongs with somebody who welds for a living.
What it costs
A single localised patch — a sill section or a small floor repair — is typically £250 to £500, and nearly all of that is labour, since the steel itself costs very little. Cost scales with how much has to be cut out, how much has to be removed to reach it, and whether the fuel tank, exhaust or interior trim must come out first. Both sills, or a chassis rail and outrigger on a vehicle with a separate chassis, reaches the top of the band and past it. Get the car properly assessed before committing: a good welder will tell you what they expect to find once they start cutting, and on a car worth £1,500 the honest answer is sometimes not to start at all.
What happens if you ignore it
Corrosion never stops on its own; it works outwards from inside a box section, so what you can see is always the smallest part of it. In a prescribed area the consequence is that a seatbelt anchorage or a suspension mounting tears out under exactly the load it exists to carry — a crash, or a heavy pothole at speed. Financially this is the defect most likely to end a car's life: a repair that would have been £300 last year is £1,200 this year, and at some point it exceeds what the car is worth, which is how most older cars in Britain finally leave the road.
Common questions
- Does rust fail an MOT?
- Where the rust is matters far more than how much of it there is. Corrosion within a prescribed area that has weakened the structure is a major defect and a failure, judged by the tester using finger and thumb pressure or a corrosion assessment tool. Elsewhere the bar is lower: a holed floor or sill that does not weaken the structure may only be a minor defect or an advisory.
- What is a prescribed area on an MOT?
- Any area within 30cm of a seatbelt anchorage, a suspension or steering mounting, a brake component mounting or a body-to-chassis mounting. These are the points that carry load in a crash or over a pothole, so corrosion there is assessed far more strictly than rust on a wing or a door, and severe weakening is graded dangerous.
- Can you use filler or a riveted plate to pass an MOT?
- No. Within a prescribed area the repair must be continuously seam welded and at least as strong as the original panel. Filler, glass fibre, pop-riveted plates and a coat of underseal over the top are not acceptable, and a tester who finds a previous repair of that kind will fail it. Poor past repairs laid over rust that was never cut out are common.
- How much does welding cost for an MOT?
- A single localised patch, such as a sill section or a small floor repair, is typically £250 to £500, and nearly all of that is labour because the steel itself costs very little. Cost scales with how much has to be cut out and how much has to be removed to reach it. Both sills, or a chassis rail and outrigger, reaches the top of the band and past it.
- Is it worth repairing structural rust?
- Sometimes not, and it is worth being honest about that before committing. Corrosion works outwards from inside a box section, so what you can see is always the smallest part of it, and a repair that would have been £300 last year is £1,200 this year. On a car worth £1,500 the sums stop working, which is how most older cars in Britain finally leave the road.
Cars this happens to most
Of every model with at least 2,000 tests in the DVSA data, these record corrosion in a structural area on the largest share of their MOTs. Across all of them the typical rate is 1.6% of tests.
- Mazda Bongo15%
1,318 times in 8,769 tests
- Suzuki Jimny13%
3,021 times in 23,408 tests
- Suzuki Carry11%
388 times in 3,464 tests
385 times in 3,513 tests
- Iveco Daily11%
311 times in 2,951 tests
- Talbot Express9.9%
478 times in 4,826 tests
- Toyota Hiace8.4%
517 times in 6,129 tests
- Isuzu Rodeo8.4%
350 times in 4,178 tests
A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.