Corrosion and body faults at the MOT by car age: what to look at underneath before the salt goes down

The short answer

Across models with 500 or more tests, about 1 in 500 first tests on cars first used in 2021 (0.2% of 2,021,332 tests) failed with a body, chassis or structure item, in tests carried out 2021 to 2025. For cars first used in 2006 it was about 1 in 22, or 4.5%. The line between is one of the steepest in the data: 0.5% for 2018 cars, 1.3% for 2015, 2.5% for 2012, 3.4% for 2009. As a share of the fails that happen, the family goes from 2.8% on 2021 cars to 12.4% on 2006 cars.

But look at what the heading contains. The DVSA grouping puts the exhaust system and the rubber boots on the driveshaft joints in with the bodywork and structure, and those two account for most of the fails at every age. Corrosion severe enough to fail is rarer: structural corrosion on 0.6% of first tests on cars aged 15 and over, chassis corrosion on 0.4%. Corrosion shows up far more often as an advisory, on 1.5% of tests at 15 and over for the chassis and 1.5% for the structure. The advisory is the moment to act, and the weeks before the gritters go out are the time to look.

Body and structure fails by item and car age

Each cell is the share of all first tests in that age band that failed with the item, pooled across 2021 to 2025. A dash means the item was outside the eight most recorded for that band.

The driveshaft boot leads every band. It is a rubber bellows that keeps grease in and dirt out of the joint behind each front wheel; once it splits, the grease sprays over the inside of the wheel and the joint starts to wear. The exhaust rows are the next two at every age. The corrosion rows do not appear at all until the 10 to 14 band and only become a regular entry past fifteen.

Corrosion is an advisory long before it is a fail

The same store records what the tester noted but did not fail. On cars aged 15 and over, a chassis corrosion advisory was recorded on 1.5% of first tests and a structure corrosion advisory on 1.5%, against 0.4% and 0.6% as fails. On cars aged 10 to 14 the advisory figures are 0.3% for each, with the sub-frame noted on 0.2%. Exhaust leaks follow the same pattern: an advisory on 4.9% of tests at 15 and over, 2.6% at 10 to 14 and 0.8% at 6 to 9. A corrosion advisory means the tester saw rust that had not yet reduced the strength of the part. A winter of salt is what moves it from one list to the other.

What the tester is looking for

The DVSA inspection manual is clear that surface rust is not the test. The structure fails as a major defect when it is corroded to the extent that the rigidity of the assembly is significantly reduced, and as a dangerous one when steering or braking is likely to be affected. The tester assesses corrosion within 30cm of the mountings where the body meets the chassis, and the manual says a vehicle should only be failed where overall structural rigidity is significantly reduced. A body panel is a fail only if it is likely to cause injury when touched, or is insecure. So the sills, the jacking points, the rear spring seats and the subframe mountings matter; a bubbling wheel arch does not, unless the edge is sharp.

What to look at before the salt goes down

1. Wash underneath after any run on gritted roads, and once now while the underside is still dry and visible.
2. The driveshaft boots. Turn the steering to full lock and look behind each front wheel. A spray of dark grease on the inside of the wheel means a split boot.
3. The exhaust. Start the car cold and listen for a blow. Push the system gently from underneath: hangers that have perished let it move and knock.
4. Sills and jacking points. Press with a thumb along the inner sill and at the jacking points. Soft, flaking or bubbling metal there is what the manual's rigidity wording is about.
5. Rear spring seats and subframe mountings. On older cars these are where the 15-and-over corrosion figures live. A torch and a mirror will do.
6. Drain holes. Clear the slots along the bottom of each door and sill so water gets out.
7. Last year's advisories. The free MOT history service shows whether a corrosion note was already on the car. If it was, it is a winter older now.

Doors, bonnet, bootlid and tailgate are on GOV.UK's list of items retested free the next working day at the same centre; the exhaust, the driveshaft boots and the structure are not, so a fail on those means a partial retest within 10 working days, for which a fee may be charged. A car failed on a dangerous item cannot be driven away except for repair: up to £2,500, three penalty points and a possible ban.

What this does not mean

These are fleet figures across every model with 500 or more tests. They say that body and structure fails are rare under ten years and mostly exhausts and boots at every age; they do not say any make rusts, or that a car of any age is sound. The store records what was found on the day and not why: where a car is kept, how often it is washed, whether it lives near the coast or on gritted hills, and whether a previous owner treated the underside are not in it. A sixteen-year-old car garaged and washed can carry nothing on this list; a six-year-old car left on a salted road can carry an advisory already.

Sources and method

Cohort shares are class 4 first tests, 2021-2025, across models with 500 or more tests, by first-used year (4,746,066 tests first used 2006 to 2,021,332 first used 2021), shown as the share of all first tests that failed with at least one item in the body, chassis and structure family, which in the DVSA grouping used here also contains the exhaust system and driveshaft joint boots. Fail and advisory item shares are each item as a share of all first tests in the age band (33,793,352 tests aged 3-5, 50,103,915 aged 6-9, 44,980,002 aged 10-14, 31,405,150 aged 15 and over). A test can record several items, so rows do not add up; a dash means the item was outside the eight most recorded fails for that band. National context: 72.3% of 33,210,951 first tests passed in 2025.

• DVSA anonymised MOT results, class 4 first tests, 2021-2025 (data.gov.uk)
• GOV.UK: MOT inspection manual, section 6, body, structure and attachments
• GOV.UK: MOT retests after a repair
• GOV.UK: MOT test result
• GOV.UK: Check the MOT history of a vehicle

This article is general information drawn from published test records. It is not professional, mechanical or legal advice, and an MOT pass or a clean MOT history does not guarantee that a car is roadworthy today. If you are unsure about a car, have it inspected by a qualified mechanic. You can check any vehicle's MOT history free on the official GOV.UK MOT history service.