Corrosion on the body or underbody
Last reviewed 22 October 2026. General information, not advice about a particular car.
Keep an eye on it Normal wear or minor. Check it at the next service.
"Corroded but not seriously weakened" means rust is present but the structure is still sound. On older cars this is routine, but it only goes one way: once rust reaches a prescribed area or weakens a mounting it becomes a fail, and welding is costly. Check how far it has spread.
What the wording on the certificate means
You’ll usually see something like “Corroded but not seriously weakened”, “Underbody corroded but not seriously weakened” or “Sill corroded but not seriously weakened”, often with a position such as “nearside front” (the passenger side) and a reference to the MOT inspection manual in brackets.
The tester found rust on part of the body or underbody, such as a sill, floor pan or subframe, but it hasn’t weakened the structure yet.
Why it matters
Rust spreads. The MOT pays most attention to load-bearing areas near the mounting points for the suspension, steering and seat belts. Rust there that weakens the structure is a fail, and repairing it usually means cutting out and welding in new metal.
What to do
- Ask exactly where the rust is and whether it’s near a mounting point.
- Surface rust on a subframe is common and can be cleaned back and treated with rust converter and underbody wax or underseal.
- Rust on sills and floors is worth treating early, before it goes through.
Will it fail the next MOT?
An advisory isn’t a fail, and the car can be driven, as long as it stays roadworthy. Corrosion that seriously weakens the structure in those load-bearing areas is a major fail, and dangerous if it affects the steering or brakes.
If you’re buying a car with this advisory
Repeated corrosion advisories on an older car are a warning sign. Look underneath, or pay for an inspection before you buy. If the same advisory shows up at test after test, it has probably been left unrepaired.
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