MOT advisory explained
“Corrosion within 30cm of a mounting”
Every other advisory on your certificate comes with a price. This one comes with a measurement instead — and until somebody scrapes the rust back, nobody in the country can tell you what it costs.
Rust has been found in the prescribed area around a mounting point for the suspension, steering, brakes or a seatbelt. The metal is still sound enough to pass — but corrosion in this zone is assessed against structural standards rather than cosmetic ones.
The distance is the point. The same patch of rust 40cm from a mounting is not an MOT item at all. Within 30cm, it holds part of what holds the car together.
No range is given here because none would be honest. Surface scale on a sound section and a rotten sill both read the same on a certificate, and they are not the same repair.
Do not accept a price over the phone for this one. Anybody quoting without having seen bare metal is guessing, and the guess can be wrong by an order of magnitude in either direction.
A “corrosion within 30cm of a mounting” advisory means rust has been found in the prescribed area around a suspension, steering, brake or seatbelt mounting point, but the metal is still sound enough to pass. Nobody can price the repair until the rust is scraped back. Of 430,000 tracked structural advisories, 6.7% later returned as a failure.
Why 30cm, and why it changes everything
Most of a car’s bodywork is not an MOT item. A rusty wing, a bubbling door bottom, a corroded boot floor — none of it is tested, because none of it holds anything that matters when you brake or steer. Then the tape measure comes out, and the rules change completely.
- A mounting point is where something structural bolts on. Suspension arms and subframes, steering rack and column, brake components, and seatbelt anchorages. These are the parts that transmit force into the body shell, and the shell has to be able to take it.
- Within 30cm of one of those points, the metal is assessed as structure. The tester is not judging whether it looks bad. They are judging whether it could still carry load — which is a completely different question and a much stricter one.
- Outside that zone, the same rust is not an MOT item. It might be ugly, it might spread, it might eventually matter. But on the day of the test it is not what is being examined, which is why cars with visibly scabby arches sail through while a small patch you cannot see gets written up.
- Testers assess it by feel as much as by eye. The standard method is scraping and tapping. Sound metal rings and resists; corroded metal sounds dull and gives. A tester is allowed to scrape away loose material to see what is underneath, which is exactly what a quoting garage has to do too.
So the advisory is telling you two things at once: there is rust, and it is somewhere the rules care about. Those two facts together are what make this different from every other corrosion on the car.
Why nobody can price this one
Every other page on this site opens with a range. Brake pads, £90–£220. A pair of dampers, £150–£360. Even an oil leak, where the spread runs from £150 to £800, has a number at each end. This page does not, and the reason is worth understanding rather than working around.
Corrosion hides its own extent. Rust works from the inside of a panel outwards and from the back of a seam forwards, so what is visible is the end of the process rather than the beginning. A patch the size of a coin can sit on top of sound steel, or it can be the only part of a much larger hole that has broken through to the surface yet.
Nobody knows which until it is scraped back. That is not a garage being evasive. It is the actual state of the information: until loose material is removed and the remaining metal is tapped, the extent of the repair is genuinely unknown to everybody involved, including the tester who wrote the advisory.
Which is why an inspection is the first job, not the quote. Half an hour with a scraper converts an unknown into a defined repair, and it is the only sequence that gives you a number you can trust. A quote produced without it is a guess wearing a price tag.
And occasionally the inspection produces an answer no repair can improve on. Structural corrosion is the one advisory in the test where the finding can be that the car is not economic to save — not because the welding is impossible, but because the metal that would have to be replaced is worth more than the car. That possibility is why this page has no range and why it is worth reading before you book anything.
When it stops being an advisory
The threshold is whether the structure is significantly weakened. Corrosion in the prescribed area fails when it has reduced the metal’s ability to do its job — typically where it has holed through, where the section has thinned to the point of giving under a tap, or where a seam or repair has come apart.
Surface rust in the same place is not a failure. Scale on top of solid metal is what an advisory here usually describes, and it is why the car passed. The difference between the two is exactly the thing a scrape reveals and a photograph does not.
Previous repairs are assessed too. A welded patch in a structural area is legitimate, but it has to be a proper repair — continuous where it should be continuous, in the right material, and sound. Riveted plates, filler over holes and glass-fibre patches in a load-bearing zone do not pass, and a tester who finds one usually looks harder at everything around it.
The consequence of crossing the line is unlike anything else on the certificate. A worn pad means a pad. A rotten mounting means a body shell that cannot be trusted to hold a suspension arm, and there is no small version of that repair.
How often does this advisory become a failure?
This can be measured rather than guessed. Following every structural advisory forward to the same vehicle’s later tests, across 430,000 of them, shows what actually happened.
| What happened next | Structure | Suspension | Oil leak |
|---|---|---|---|
| Became a failure at a later test | 6.7% | 18.7% | 1.3% |
| Ignored, no action taken | 17.5% | 37.1% | 43.5% |
| Fixed | 75.9% | 44.2% | 55.1% |
| Median advisory to failure | 384 days for structure — the next MOT | ||
From the DVSA MOT testing bulk dataset (Open Government Licence v3.0): 829.7 million tests and 145.1 million advisories tracked to each vehicle’s later tests, from 429,858 structural advisories. Defect code 6.2 covers the vehicle structure as a whole, not corrosion near mountings alone.
Three in four of these get repaired, and fewer than one in five is left alone — the highest repair rate and the lowest neglect rate of any category we have measured. It is also the rarest advisory in the record, written a fiftieth as often as a tyre.
If drivers repaired advisories according to how likely they were to fail, this one would sit in the middle. Its conversion rate, 6.7%, is a third of a suspension advisory’s. Yet suspension is the least repaired thing in the record at 44.2%, and structure is the most repaired at 75.9%.
What separates them is not probability but consequence. A suspension advisory that goes wrong costs a few hundred pounds. A structural one that goes wrong can end the car. People are weighing how bad the worst outcome is, not how likely it is — and on this advisory that instinct is sound, because the worst outcome is the only one that cannot be undone by spending money.
It is a different mechanism again from a wheel bearing, repaired almost as often simply because it hums at you every day. Noise, probability and consequence each drive behaviour, and they are not the same lever.
How the risk changes with the age of the car
Broken down by how old the car was when the advisory was written, this group climbs more steeply than anything else in the record.
| Age of car at the advisory | Advisories | Became a failure |
|---|---|---|
| 2 years | 2,599 | 1.7% |
| 7 years | 8,019 | 2.9% |
| 12 years | 27,257 | 5.6% |
| 17 years | 32,796 | 7.4% |
| 22 years | 10,454 | 9.4% |
Same dataset, broken down by vehicle age at the time of the advisory. Figures beyond the mid-twenties rest on small numbers and are not shown. Contains public sector information licensed under the Open Government Licence v3.0.
The risk rises more than fivefold between a two-year-old car and a twenty-two-year-old one. That is a steeper climb than any other group, and it makes sense once you notice what corrosion does not do: it never pauses, never reverses, and never benefits from the car being looked after in any way that undoes what has already happened.
It is the same pattern as an exhaust leak, and for the same reason. Wear items behave differently — a wheel bearing peaks around fifteen years and then declines, because very old cars on the road are survivors. Rust does not care whether a car is a survivor. Every winter adds to a total that only goes one way.
What drives the cost, since we cannot give you a figure
| What decides the price | Cheap end | Expensive end |
|---|---|---|
| Extent of the corrosion | Surface scale on sound metal | Holed through, spreading under seams |
| Where it is | Open, accessible panel | Boxed section or behind a subframe |
| What has to come off | Nothing | Fuel tank, suspension, interior trim |
| Complexity of the section | Flat plate | Curved, seamed or double-skinned |
These four factors, not the make or model, are what separate a modest welding bill from one that exceeds the car’s value. That is why the inspection has to come first.
Welding is the only real repair. Corroded metal in a structural area is cut out and new steel welded in, seam-welded where the original was. Filler, plating over the top and fibreglass are not repairs in this zone — they are failures waiting to be found, and a tester who spots one has good reason to look at everything nearby.
Get the inspection separately from the quote, and get it in writing. Ask for the extent described: how far the corrosion runs, whether it has holed through, what has to be removed to reach it. That description is what lets you take the job to a second garage without paying for a second investigation.
Ask the awkward question early on an older car. If the inspection suggests the repair approaches what the car is worth, that is worth knowing before the welder is booked rather than after. It is not a comfortable conversation, but it is a cheaper one than finding out halfway through.
Rust proofing does not undo anything. Treating and sealing an area slows what has not happened yet; it does not restore metal that has gone. It is worth doing on the sound areas around a repair, and worth nothing as a substitute for one.
Enter your reg — we’ll read your MOT history, decode each advisory and price it for your exact model.
What to do next
- Book an inspection, not a repair. The first job is finding out how much metal is actually gone. Everything else — the quote, the decision, the timing — depends on an answer nobody has yet.
- Ask exactly where it is and how far it runs. A written description of the extent lets you get a second opinion without paying twice to have the same rust scraped.
- Check your MOT history for how long it has been mentioned. A first appearance and a third appearance are different situations: corrosion never stops, so three years of the same note means three more winters of it.
- On an older car, ask what it would cost against what the car is worth. This is the one advisory where the honest answer is sometimes not to repair — and finding that out before the work starts is worth a great deal.
Structural corrosion advisory FAQ
What does corrosion within 30cm of a mounting mean?
Rust has been found within the prescribed area around a point where something structural bolts to the body — a suspension component, the steering, a brake part or a seatbelt anchorage. Inside that zone the metal is assessed as structure rather than bodywork, so the standard is whether it can still carry load, not whether it looks acceptable.
Why does the MOT care about 30cm specifically?
Because that is the area where corrosion affects whether the body can hold what is bolted to it. Outside the prescribed zone, rust is not an MOT item at all — which is why cars with visibly rusty arches pass while a small patch near a subframe mounting gets written up. The distance decides whether the rust counts, not its appearance.
How much does it cost to weld corrosion near a mounting?
There is no honest range, and that is not evasion. Corrosion hides its own extent — rust works outwards from inside a panel, so a patch the size of a coin can sit on sound steel or be the visible corner of a much larger hole. The cost is decided by how far it runs, how accessible it is and what has to come off to reach it, none of which anybody knows until the area is scraped back.
Will corrosion near a mounting fail my next MOT?
It fails when the corrosion has significantly weakened the structure — where it has holed through, thinned enough to give under a tap, or where a previous repair has come apart. Surface scale on solid metal is what an advisory usually describes. Since corrosion never reverses, the risk rises every year: from 1.7% on a two-year-old car to 9.4% on one at twenty-two.
Can rust near a mounting be patched or filled instead of welded?
No. In a structural area the only acceptable repair is cutting out the corroded metal and welding in new steel, seam-welded where the original was. Filler, riveted plates and glass-fibre are not repairs there, they fail the test on their own terms, and a tester who finds one has good reason to examine everything around it.
Is it safe to drive with structural corrosion?
At the advisory stage the tester judged the metal still able to do its job, which is why the car passed. What makes this one worth acting on is not the risk today but the nature of the worst outcome: a mounting that lets go is not a repair, it is a failure of the thing holding the suspension to the car.
Can corrosion make a car not worth repairing?
Yes, and this is the one advisory where that is a realistic outcome. The welding itself is rarely impossible — the question is whether the labour to reach and replace the metal exceeds what the car is worth. On an older vehicle it is worth asking that before booking the work rather than discovering it partway through.
How often does a structural corrosion advisory become an MOT failure?
Across 430,000 tracked structural advisories, 6.7% returned as a failure at a later test, 75.9% were repaired and 17.5% were ignored — the highest repair rate and lowest neglect rate of any category we have measured. Owners are not responding to the odds, which are moderate, but to the consequence: this is the only advisory where the answer can be that the car is finished.