MOT advisory explained
“Structure or chassis corroded within 30cm of a mounting”
The MOT has a stricter standard for body rust close to the points that hold the suspension, steering and seat belts on. This advisory means the tester found rust inside that zone and judged it not yet serious. It is the same rust as an ordinary structure advisory, held to a higher bar.
Around every mounting point for a seat belt, a suspension component, the steering or the brakes, the tester assesses a circle thirty centimetres across. Rust inside that circle is judged more harshly than rust elsewhere. Yours is inside the circle and still sound.
4.7% fail on the structure at the next test, and only 15.2% are advised again, one of the lowest repeat rates among the corrosion advisories. Rust in a prescribed area does not get the benefit of the doubt twice.
There is a separate wording for rust at the mounting itself, under the suspension section of the test. Suspension mounting prescribed area corroded fails twice as often as this one. If your certificate has both, the mounting page is the one to read first.
A “chassis corroded within 30cm of a mounting, but not seriously affecting overall security” advisory means rust has been found inside the thirty centimetre zone around a seat belt, suspension, steering or brake mounting, where the MOT applies a stricter standard, but it is not yet serious, so the car passes. According to DVSA MOT data analysed by PriceMyRepair, 4.7% of 289,591 such advisories followed to the next test failed on the vehicle structure, and only 15.2% were advised again.
The thirty centimetre rule
Prescribed areas. The testing manual defines a prescribed area as any part of the body or chassis within thirty centimetres of where a seat belt, a suspension component, the steering, or a brake component is mounted. Those are the loads a body has to carry in a crash or an emergency stop. Rust that would be an advisory on a door step is a failure on a seat belt mounting.
Where it usually is. “sill” is the most common note by a wide margin, then “inner sill” and “outer sill”. The sills carry the front seat belt mountings at their base and the rear suspension mountings at their back end, so most of a sill is inside somebody’s thirty centimetres. “wheel arch”, “floor” and “jacking point” follow.
How the tester decides. Corrosion in a prescribed area fails if it has reduced the strength of the metal or if the tester can push a blunt tool through it. Surface rust and light scale on solid metal is this advisory. The words change from “not seriously affecting” to “seriously affecting” when the probe goes in.
Will it fail next year?
Three wordings share the page, for an integral body, a separate chassis, and a van’s cab. Together they have been recorded 1,015,743 times, and 289,591 can be followed to the same vehicle’s next MOT.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 4.7% |
| Failed on anything under defect 6.2.2 | 6.2% |
| Same advisory recorded again | 15.2% |
According to DVSA MOT data analysed by PriceMyRepair: 289,591 advisories followed to the same vehicle’s next test. Rows can overlap.
The conversion figure sits next to general structural corrosion at 4.9%, but the repeat rate is very different: 15.2% here against 25.5% for the general wording. Rust near a mounting is either treated, or it fails, or the tester reclassifies it next year under one of the other structure wordings.
Rust at a suspension mounting itself, recorded in the suspension section of the test, fails 10.2% of the time. This advisory is the ring around that centre.
| Advisory wording | Fails on same part | Advised again | Followed |
|---|---|---|---|
| Seat belt anchorage prescribed area is corroded | 12.6% | 21.0% | 194,901 |
| 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 |
| Vehicle structure is corroded but structural rigidity is not significantly reduced | 4.9% | 25.5% | 1,233,347 |
| Integral body structure or chassis corroded within 30cm of a mounting, | 4.7% | 15.2% | 289,591 |
| Suspension arm corroded | 3.0% | 35.7% | 1,384,389 |
| Sub-frame corroded | 2.0% | 39.8% | 2,445,675 |
Nothing to report under seven years. From 7–9 years it climbs from 0.2 per 1,000 tests to 11.9 at 20 years and over, and the share that fail next time rises from 1.8% to 5.8%. Body rust that has reached a mounting has usually been going for several winters.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 7–9 years | 0.2 | 1.8% | 7,410 |
| 10–12 years | 1.2 | 3.6% | 37,351 |
| 13–15 years | 3.7 | 4.4% | 79,870 |
| 16–19 years | 7.6 | 5.0% | 98,012 |
| 20 years and over | 11.9 | 5.8% | 59,441 |
This is the advisory to act on while it is still an advisory. Once a prescribed area fails, the only repair is welding a plate or a new section in, and the tester at the retest will want to see clean, continuous welds, not filler or sealant.
At the advisory stage, cleaning back to bright metal, converting the rust, sealing and cavity waxing inside the sill is a modest job and often holds the line for years.
If the certificate names the seat belt mounting specifically, have it looked at before anything else. A seat belt pulls on that point with more force than anything else in the car.
Exact wording on your certificate
The three wordings describe the same finding on three kinds of vehicle body. The failure is the same sentence with the opposite ending.
| Line on the certificate | Result | Code |
|---|---|---|
| Integral body structure or chassis corroded within 30cm of a mounting, but not seriously affecting overall security | Advisory — pass | 6.2.2 (d) (i) |
| Vehicle structure or chassis corroded within 30cm of a mounting, but not seriously affecting overall security | Advisory — pass | 6.2.2 (d) (i) |
| Cab or chassis corroded within 30cm of a mounting, but not seriously affecting overall security | Advisory — pass | 6.2.2 (d) (i) |
| Vehicle structure or chassis corroded within 30cm of a mounting, seriously affecting overall security | Major — fail | 6.2.2 (d) (ii) |
The rear corners lead, with the offside rear and nearside rear ahead of the fronts. The rear suspension mountings on most cars sit at the back of the sills and in the rear arches, exactly where mud packs and water sits.
| Position written by the tester | Share of all instances |
|---|---|
| No position recorded | 15% |
| Offside rear | 13% |
| Nearside rear | 12% |
| Offside front | 9% |
| Nearside front | 8% |
| Rear | 7% |
From 1,015,743 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “sill” (54,259), “inner sill” (14,545), “outer sill” (10,074), “wheel arch” (4,635), “floor” (3,905), “both sides” (3,525), “sills” (2,804), “chassis” (2,340).
What it costs
Treating surface rust in a prescribed area is a modest labour job and worth doing at the next service. Welding a sill or a mounting area is quoted per section and depends on access and how far the rust has gone, so we do not publish a range. Ask for a quote that names the area on the certificate. Other prices are on the repair costs index.
Can I still drive with this advisory?
There is nothing to feel or hear. Have the area treated before winter, when road salt does the damage, and do not leave it to the next test.
Enter your reg to see every corrosion advisory on your car and whether this one has moved from a general structure note to a mounting area over the years.
Which cars get this advisory most?
The all-vehicle rate is 0.9 per 1,000 tests. On the models below it is four to eleven times that.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Mitsubishi Shogun | 10 | 10.7× | 420,039 |
| Land Rover Discovery | 6 | 6.7× | 1,610,504 |
| Ford Ranger | 5 | 5.9× | 1,001,179 |
| Kia Sorento | 5 | 5.3× | 392,464 |
| Ford Ka | 4 | 5.0× | 2,120,213 |
| Ford Transit | 4 | 4.5× | 9,907,752 |
| Mitsubishi L200 | 4 | 4.4× | 935,407 |
| Toyota Corolla | 4 | 4.1× | 1,166,652 |
| Land Rover Defender | 4 | 4.1× | 1,232,451 |
| Mazda MX-5 | 4 | 4.1× | 962,840 |
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.
The Mitsubishi Shogun leads at 10.7 times the average, and 4x4s and pickups take half the list: the Land Rover Discovery, Ford Ranger, Kia Sorento, Mitsubishi L200 and Land Rover Defender. On a separate chassis the suspension mountings are welded brackets on bare steel rails, and they are the first thing to rust.
The cars are long-lived ones with known sill problems: the Ford Ka, Toyota Corolla and Mazda MX-5.
What to do next
- Find out which mounting. Seat belt, suspension, steering or brake. The certificate or the tester will say, and the seat belt one comes first.
- Look at it with a torch. Brown and solid is this advisory. Bubbling, layered or holed is next year’s failure.
- Treat it before the salt goes down. Clean, convert, seal, and wax inside the cavity. One afternoon on a ramp.
- If it needs welding, use a welder. Filler, sealant and underseal over rust are what testers are trained to find, and they will.
Corrosion within 30cm of a mounting FAQ
What does corroded within 30cm of a mounting mean on an MOT?
Rust has been found on the body or chassis inside the thirty centimetre zone around a seat belt, suspension, steering or brake mounting, where the MOT applies a stricter standard. The tester judged it not yet serious, so it is an advisory and the car passes.
What is a prescribed area?
Any part of the vehicle structure within thirty centimetres of a mounting for a seat belt, suspension component, steering or brake component. Corrosion in a prescribed area fails the MOT if it has weakened the metal, where the same rust elsewhere might only be an advisory.
Will it fail next year?
According to DVSA MOT data analysed by PriceMyRepair, 4.7% of 289,591 such advisories failed on the vehicle structure at the next test. Only 15.2% were advised again, so most are either repaired or reclassified within the year.
How is rust in a prescribed area repaired?
At the advisory stage, by cleaning, treating and sealing the metal. Once it has weakened the structure, by welding new metal in. Filler or sealant over rust is not an acceptable repair.
How much does it cost?
Treatment is a modest labour job. Welding is quoted per section and varies widely with access and extent, so we do not publish a range. Any prices on this site are estimates and final quotes come from garages.
Which vehicles get this advisory most?
The Mitsubishi Shogun, at 10.7 times the average rate, and other body-on-frame 4x4s and pickups such as the Land Rover Discovery and Ford Ranger, whose suspension mountings are welded to bare steel chassis rails.
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 1,015,743 times; 289,591 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.