MOT advisory: sub-frame corroded but not seriously weakened
A sub-frame is a separate frame bolted under the car that carries the engine, gearbox or suspension. Being low down, it collects salt and water. The tester saw rust on it but judged it hadn't seriously weakened the metal.
What the tester wrote
| Result | Wording on the certificate | Category |
|---|---|---|
| Advisory | Sub-frame: corroded but not seriously weakened | Advisory (pass) |
| Fail | Sub-frame: corroded and seriously weakened | Major |
What the MOT checks
The tester inspects the sub-frame and its mountings from underneath. Corrosion that seriously weakens it is a Major fail. Surface rust that hasn't eaten into the metal is noted instead.
The MOT also treats the body structure within 30cm of a sub-frame, spring or suspension mounting as a prescribed area. Loss of strength or a poor repair there fails the car.
"The strength or continuity of the load bearing structure within 30cm of any sub-frame, spring or suspension component mounting (a ’prescribed area’) is significantly reduced or inadequately repaired"
MOT inspection manual, section 5.3.3 Suspension arms, rods, struts, sub-frames, anti-roll bars etc.. Read it on GOV.UK
How common it is
In 2025 this advisory was recorded on 2.6 in every 100 class 4 MOT tests (854,162 of 33,210,774 initial tests). The matching fail was recorded on 0.1 in every 100 (43,417 tests).
The typical car with the advisory was 15 years old with 100,296 miles recorded (medians).
| Age at test (years) | Advisory per 100 tests | Fail per 100 tests |
|---|---|---|
| 0-2 | not shown | not shown |
| 3 | not shown | not shown |
| 4-5 | under 0.1 | not shown |
| 6-7 | 0.2 | not shown |
| 8-10 | 1.3 | 0.1 |
| 11-14 | 4.4 | 0.2 |
| 15-19 | 7.3 | 0.4 |
| 20+ | 4.7 | 0.2 |
'Not shown' means fewer than 1,000 tests in that cell, or a cell hidden so a small one can't be worked out. Age is years since first registration. First MOTs often happen just before a car's third birthday, so many show as age 2.
Does it turn into a fail at the next MOT?
We followed the same cars to their next MOT, 240 to 500 days later (521,840 pairs where this advisory was recorded). 1.9% failed on the same item at that next test, and another 45.9% had the same advisory again.
Among cars where this item wasn't noted at all, 0.1% failed on it next time. So a car with this advisory was about 19 times as likely to fail on this item as one without it, though the chance is still small.
These are averages. We can't see what owners did between tests. Some will have fixed the problem, and a fix would stop a fail.
What it usually means for a buyer
Very few cars with this note fail on it at the next test, and many get the same note again. So surface rust on a sub-frame tends to move slowly.
The useful questions are how far it's gone and where. Rust on the sub-frame is one thing. Rust spreading into the floor or the mounting points around it is more serious. If you can, look underneath with a torch, or pay for an independent inspection on an older car before you buy.
Typical fix
Light surface corrosion can be cleaned and treated. A badly corroded sub-frame is usually replaced, which is a bigger job. Welded repairs near mounting points need doing properly, because a poor repair in a prescribed area is a fail in itself. Prices vary a lot by car and garage, so get a quote for the specific car.
Questions to ask the seller
You can read any car's MOT history, including advisories, free on GOV.UK. Our MOT advisory decoder explains the other notes in plain English.
Related MOT advisories
- Rigid brake pipe corroded
- Engine oil leak, but not excessive
- All MOT defects
- Buying a used car: a step-by-step guide
Sources and limitations
- Data: DVSA MOT anonymised test results and test items, class 4 initial tests, 2024 and 2025 (open data on GOV.UK). Same DVSA data as our MOT dataset.
- 'Fail' includes items fixed at the test centre within an hour (pass after rectification).
- An advisory is a tester's judgement on the day. Testers differ, and some problems are not visible at an MOT.
- The MOT is not a service and does not check the engine, clutch or gearbox in depth.
- Counts under 1,000 are not shown. No make or model figures are published here.
Contains public sector information licensed under the Open Government Licence v3.0. Source: Driver and Vehicle Standards Agency (DVSA), MOT Anonymised Data. Analysis by CarCheckHQ. Not endorsed by DVSA.
Written by the CarCheckHQ team.