Corrosion and structural MOT failures
Rust is the failure that ends more cars' lives than any other. It's often the most expensive item on the sheet – and the one most likely to grow once the welder starts.
Checked against the MOT inspection manual and GOV.UK guidance · Updated November 2026
What the tester is looking for
The tester is not checking whether a car looks rusty. They're checking whether corrosion has weakened the structure – the parts that keep the car rigid and hold the brakes, steering, suspension and seatbelts in place. Surface rust on a wing or door bottom isn't a fail. A sill you can push a screwdriver through is.
Prescribed areas and the 30cm rule
The MOT manual defines prescribed areas: any load-bearing structure within 30cm of a mounting point for:
- steering components
- suspension components (spring seats, shock absorber towers, subframe and arm mountings)
- brake components
- seatbelt anchorages and seat mountings
Excessive corrosion in a prescribed area, or anywhere that significantly reduces the strength of a load-bearing member, is a major defect – or dangerous if the component is close to detaching. Load-bearing members include sills, floor pans, chassis legs, crossmembers and subframes.
Common corrosion failures
| Defect found | Usual category |
|---|---|
| Excessive corrosion in a prescribed area | Major |
| Corrosion so severe that a mounting could fail, affecting control | Dangerous |
| Sill or floor pan excessively corroded (load-bearing) | Major |
| Subframe excessively corroded or damaged | Major |
| Repair with filler, fibreglass or non-structural patching over a load-bearing area | Major |
| Sharp edges or projections likely to cause injury | Major |
| Door, bonnet or boot unable to close securely | Major |
| Body or cab insecure / likely to become detached | Major / Dangerous |
Bodged repairs
Testers tap and scrape suspect areas. Filler, fibreglass, mesh or riveted plates over a structural hole are all fails – structural repairs must be welded with suitable steel, continuously seamed. Expect a tester to fail a patch that's been tacked on top of rotten metal.
Why welding quotes grow
Corrosion spreads behind the visible hole. Once the welder cuts back to clean metal, the repair is often bigger than the quote assumed. Ask your garage for a firm price or a ceiling, and for photos before and after. Also ask whether other areas the tester listed as advisories will need welding next year.
Repair costs
| Repair | Typical cost | DIY? |
|---|---|---|
| Small structural patch (per area) | £100–£300 | Only with welding skills |
| Sill section (per side) | £250–£600 | Specialist |
| Full outer sill (per side) | £400–£900 | Specialist |
| Floor pan section | £200–£600 | Specialist |
| Rear spring seat / turret repair | £250–£700 | Specialist |
| Subframe replacement | £400–£1,200 | No |
| Welding labour rate | £50–£80/hour | — |
Welding costs vary hugely with access and how far rot has spread. Get a firm price or a ceiling in writing.