MOT advisory explained
“Sub-frame corroded but not seriously weakened”
The most repeated advisory in the suspension section. Cars pick it up at around ten years old and most carry it, unchanged, to the end of their lives. Here is what a subframe is, where rust on it counts, and what the numbers say.
A subframe is a steel cradle bolted under the car that carries the engine or the rear suspension. The tester found rust on it, probed it, and judged that it has not lost strength.
2.0% of these fail on the subframe at the next test. 39.8% are written down again exactly as before. For most owners this advisory becomes a permanent line on the certificate and nothing more.
Do not mix this up with corrosion near a suspension mounting. That line is about the body shell the subframe bolts to. It fails five times as often and cannot be unbolted and replaced.
A “sub-frame corroded but not seriously weakened” advisory means the tester found rust on the bolted steel cradle that carries the engine or rear suspension, and judged it still strong, so the car passes. No repair is required. According to DVSA MOT data analysed by PriceMyRepair, 2.0% of 2.4 million such advisories followed to the next test failed on the subframe, while 39.8% were given the same advisory again.
What a subframe is and why it rusts
A cradle under the car. Most cars have one at the front, holding the engine, steering rack and lower suspension arms, and many have a second at the rear for the suspension. They are made of pressed and welded steel, painted once at the factory, and spend their lives in road spray.
Thick metal, slow rust. Subframes are built heavier than the body. Surface rust and even scaling can go on for many years before the steel is thin enough to matter, which is why the advisory is so common and the failure so rare.
Where to look. The flat surfaces do not matter much. The points that do are the four mountings where the subframe bolts to the body, and the welded brackets that hold the suspension arms. Rust that swells a seam or eats into a bracket is the kind that eventually fails.
Will it fail next year?
With 6,839,278 recorded instances this is one of the most common advisories of all, and 2,445,675 of them can be followed to the next MOT on the same vehicle.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 2.0% |
| Failed on anything under defect 5.3.3 | 3.7% |
| Same advisory recorded again | 39.8% |
According to DVSA MOT data analysed by PriceMyRepair: 2,445,675 advisories followed to the same vehicle’s next test. Rows can overlap.
Two things stand out against other corrosion advisories. The failure rate is low: corroded brake pipes fail 10.1% of the time and a corroded coil spring 5.0%. And the repeat rate is high. Only brake pipes are re-advised more often.
That combination describes a part that rusts visibly and fails slowly. Testers are obliged to note it, year after year, and they are right to. It just does not often go anywhere.
| Advisory wording | Fails on same part | Advised again | Followed |
|---|---|---|---|
| Suspension component mounting prescribed area is corroded | 10.2% | 24.2% | 1,521,106 |
| Brake pipe corroded, covered in grease or other material | 10.1% | 44.4% | 7,889,073 |
| Coil spring corroded | 5.0% | 39.1% | 4,780,118 |
| Suspension arm corroded | 3.0% | 35.7% | 1,384,389 |
| Sub-frame corroded | 2.0% | 39.8% | 2,445,675 |
| Macpherson strut corroded | 1.6% | 22.5% | 370,013 |
| Suspension component corroded | 0.4% | 27.4% | 282,271 |
Few advisories are tied to age as tightly as this one. It is close to absent at 5–6 years, at 0.8 per 1,000 tests. On cars in their late teens it appears on nearly one test in twenty. The failure rate stays at around two in a hundred from seven years onwards and peaks at 2.2%.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 5–6 years | 0.8 | 0.5% | 25,587 |
| 7–9 years | 6.4 | 1.6% | 252,560 |
| 10–12 years | 22.4 | 2.2% | 688,348 |
| 13–15 years | 38.9 | 2.2% | 798,064 |
| 16–19 years | 46.3 | 1.9% | 534,100 |
| 20 years and over | 34.0 | 1.6% | 139,344 |
If your car is over twelve years old, this advisory says it is a normal car of its age. It is not a sign the car has been neglected and not a reason to sell it.
It does matter if you are deciding whether to spend serious money on the car. A subframe that is scaling heavily will need replacing one day, and on some models a replacement costs more than the car is worth.
Testers add “heavily corroded” to this advisory 9,270 times in the record. If that is on yours, have the mountings and brackets inspected properly on a ramp this year.
Exact wording on your certificate
The same finding can appear under two codes. One treats the subframe as part of the suspension, the other as part of the vehicle structure. They mean the same thing.
| Line on the certificate | Result | Code |
|---|---|---|
| Sub-frame corroded but not seriously weakened | Advisory — pass | 5.3.3 (b) (i) |
| Sub-frame is corroded but structural rigidity is not significantly reduced | Advisory — pass | 6.1.1 (c) (i) |
| Sub-frame damaged but not seriously weakened | Advisory — pass | 5.3.3 (b) (i) |
| Sub-frame corroded and seriously weakened | Major — fail | 5.3.3 (b) (i) |
Testers record front or rear, not a side, because a subframe spans the car. The rear is noted more often than the front. Some of the rear entries are beam axles, which testers also file here: “axle” is the single most common note.
| Position written by the tester | Share of all instances |
|---|---|
| Rear | 55% |
| Front | 38% |
| No position recorded | 4% |
| Central rear | 1% |
| Offside front | 0% |
| Central front | 0% |
From 6,839,278 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “axle” (61,611), “front and rear” (57,928), “both” (16,785), “rear axle” (14,839), “and rear” (13,421), “axle beam” (13,412), “front & rear” (12,337), “heavily corroded” (9,270).
Is there anything to pay for?
Not at the advisory stage. We do not publish a price for subframe replacement because it varies too widely by model to give an honest range. If you intend to keep the car, ask your garage to price cleaning and coating the subframe at the next service. Prices for the suspension parts that bolt to it are on the suspension repair cost page.
Can I still drive with this advisory?
There are no symptoms to watch for. A subframe that really is failing shows up as a clunk when pulling away or braking, steering that pulls, or uneven tyre wear that alignment cannot cure. None of that comes from surface rust.
Enter your reg to see when this advisory first appeared on your car and whether the tester’s wording has changed since.
Which cars get this advisory most?
The average vehicle gets this advisory on 15.9 of every 1,000 tests. These get it around three to four times as often.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Mazda 3 | 64 | 4.0× | 1,145,262 |
| MINI (all models) | 61 | 3.9× | 3,449,907 |
| Mazda 6 | 58 | 3.7× | 728,629 |
| Saab 9-3 | 53 | 3.4× | 589,429 |
| Land Rover Freelander | 47 | 2.9× | 1,480,990 |
| Mazda MX-5 | 46 | 2.9× | 962,840 |
| Jaguar XF | 44 | 2.8× | 969,012 |
| Fiat Punto | 42 | 2.6× | 1,031,944 |
| Peugeot 107 | 42 | 2.6× | 1,292,695 |
| Ford Mondeo | 42 | 2.6× | 2,546,739 |
Model families are grouped by the first word of the DVSA model name, so “Transit” covers every Transit variant. Only models with at least 60,000 tests are ranked.
Mazda takes three of the top six places with the Mazda 3, Mazda 6 and Mazda MX-5, as it does for corroded suspension arms. The MINI (all models) entry covers every MINI model, on the strength of nearly three and a half million tests.
The rest, the Saab 9-3, Land Rover Freelander, Jaguar XF and Ford Mondeo, are cars with large separate subframes at both ends. More subframe, more to rust.
What to do next
- Find out front or rear. The certificate normally says. If it says axle, it is a rear beam and not a true subframe.
- Ask about the mountings. That is the only question that separates cosmetic rust from the start of a problem.
- Protect it if you are keeping the car. Cleaned back and coated once, a subframe will usually outlast the rest of the car.
- Factor it in if you are buying. On a used car this line is normal. “Heavily corroded” on several tests in a row is a reason to look underneath before paying.
Sub-frame corroded advisory FAQ
What does sub-frame corroded but not seriously weakened mean?
The tester found rust on the subframe, the steel cradle bolted under the car that carries the engine or the rear suspension, and judged that it is still strong. It is an advisory and the car passes.
Will a corroded subframe fail the MOT next year?
Rarely. According to DVSA MOT data analysed by PriceMyRepair, 2.0% of 2.4 million such advisories failed on the subframe at the next test. 39.8% were given the same advisory again.
Is a corroded subframe dangerous?
Not while it is only an advisory. It becomes a safety issue when the rust has seriously weakened it, particularly at the body mountings or the suspension arm brackets, and at that point it fails the MOT.
Can a corroded subframe be repaired?
A subframe is bolted on, so a badly corroded one is normally replaced and not welded. At the advisory stage nothing needs repairing. Cleaning and coating it slows the rust.
How much does a subframe cost to replace?
It varies too much by model to give an honest range, from a modest job on a common hatchback to more than the value of an older car. Any prices on this site are estimates and final quotes come from garages.
Which cars get the sub-frame corroded advisory most?
The Mazda 3, at 4.0 times the average rate, then MINI models, the Mazda 6 and the Saab 9-3. It is uncommon on any car under seven years old.
How these figures were calculated. According to DVSA MOT data analysed by PriceMyRepair, from the DVSA anonymised MOT test results (Open Government Licence v3.0): 350,943,111 tests. This advisory was recorded 6,839,278 times; 2,445,675 of those were followed to the same vehicle’s next test between 180 and 730 days later. “Fails on the same part” means a failure at that next test under the same defect code and naming the same component, on either side of the vehicle. Testers do not always name a component the same way twice, so the figure is a floor. Method and how to cite.