MOT advisory explained
“Shock absorbers has slight external damage to the casing”
The word on the certificate is damage. The word testers type next to it, a third of a million times, is corroded. This is a rust advisory with a misleading name, and most of the time it needs nothing doing.
The outer tube of a shock absorber is marked, dented or rusty. The tester checked that it is not leaking and still damps, and recorded the condition of the casing as something to keep an eye on.
4.3% of these fail on the same shock absorber at the next test, among the lowest figures for any suspension advisory. A shock absorber is a sealed steel tube with thick walls, and surface rust takes years to get through it.
Per axle, fitted, independent garage. There is no repair for a casing. The shock is either left alone or replaced, and replacement is only needed once it leaks or the rust reaches a mounting.
A quote for new shocks on the strength of this line alone is premature. The line that means a shock is on its way out is “light misting of oil”, and even that one is a pass.
A “shock absorbers has slight external damage to the casing” advisory means the outer body of a shock absorber is rusty, dented or marked but it is not leaking and still works, so the car passes. No repair is needed yet. If the shock later fails, a pair costs £150–£360 per axle at an independent garage. According to DVSA MOT data analysed by PriceMyRepair, 4.3% of 452,485 such advisories failed on the same shock absorber at the next test.
Damage that is mostly rust
What testers actually mean. The standard wording says damage. The note testers add most often is “corroded”, 344,566 times, and most of the other common notes are versions of the same word. The testing system has no separate line for a rusty shock absorber, so this one is used.
When a dent matters. A dent in the outer tube of a twin-tube shock is usually harmless, because the working cylinder is inside it. A dent deep enough to touch the inner tube makes the shock stiff or jerky, and you would feel that on the road. If the ride is normal, the dent is cosmetic.
The one place rust matters. On a strut, the spring sits on a steel pan welded to the casing. If that pan rusts through, the spring can drop. Ask the garage to look at the spring seat specifically. On a separate rear shock there is no spring seat and the casing is just a tube.
Will it fail next year?
Of 1,443,236 times this advisory has been written, 452,485 could be matched to the same vehicle’s following MOT.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 4.3% |
| Same advisory recorded again | 23.2% |
According to DVSA MOT data analysed by PriceMyRepair: 452,485 advisories followed to the same vehicle’s next test. Rows can overlap.
That is about half the rate of the other shock absorber advisories. Light misting of oil fails 7.9% of the time and a worn mounting bush 8.9%, because those describe a part that has started to stop working. A rusty casing describes a part that still works and looks bad.
Nearly a quarter come back with the same line a year later. It is the kind of advisory that follows a car around.
| Advisory wording | Fails on same part | Advised again | Followed |
|---|---|---|---|
| Shock absorbers has a slightly worn bush | 8.9% | 19.9% | 607,181 |
| Shock absorbers has light misting of oil | 7.9% | 26.2% | 5,360,491 |
| Shock absorbers has a reduced damping effect | 6.8% | 13.8% | 252,845 |
| Coil spring corroded | 5.0% | 39.1% | 4,780,118 |
| Shock absorbers has slight external damage to the casing | 4.3% | 23.2% | 452,485 |
| Suspension arm corroded | 3.0% | 35.7% | 1,384,389 |
| Macpherson strut corroded | 1.6% | 22.5% | 370,013 |
Rust needs time, and the age table shows it. The advisory is almost unknown at 5–6 years, at 0.5 per 1,000 tests. From ten years on it is ten times as common. The failure rate barely reacts: 2.9% at the young end, 4.7% at its highest, at 13–15 years.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 5–6 years | 0.5 | 2.9% | 13,635 |
| 7–9 years | 1.7 | 4.1% | 65,319 |
| 10–12 years | 4.0 | 4.6% | 125,400 |
| 13–15 years | 6.0 | 4.7% | 129,901 |
| 16–19 years | 6.9 | 4.3% | 86,891 |
| 20 years and over | 5.4 | 3.4% | 25,754 |
The thing to watch for is the rust advisory and the oil advisory appearing together. Rust that has pitted the casing near the top seal lets the seal weep, and a shock that is both rusty and misting is nearer the end than either line suggests alone.
If your certificate has only this line and the shock is dry, there is nothing to do but look again next year.
Testers sometimes add “heavily” to the note. That word is worth taking seriously on a strut, because of the spring seat.
Exact wording on your certificate
Casing damage and oil leaks share a defect code, since both are about the body of the shock. The failure wording is about function, not appearance.
| Line on the certificate | Result | Code |
|---|---|---|
| Shock absorbers has slight external damage to the casing | Advisory — pass | 5.3.2 (b) |
| Shock absorber damaged to the extent that it does not function | Major — fail | 5.3.2 (b) |
Two thirds of instances are at the rear. Rear shocks sit in the spray from the front wheels, are rarely shielded, and on most cars are plain steel tubes with a thin coat of paint.
| Position written by the tester | Share of all instances |
|---|---|
| Offside rear | 27% |
| Nearside rear | 26% |
| Rear | 15% |
| Offside front | 10% |
| Nearside front | 9% |
| Front | 5% |
From 1,443,236 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “corroded” (344,566), “corrosion” (39,623), “both” (38,558), “both corroded” (35,834), “casing corroded” (28,307), “both sides” (24,672), “both sides corroded” (13,104), “rusty” (8,659).
What it costs if it ever needs fixing
| Job | Price fitted | Notes |
|---|---|---|
| Pair of shock absorbers, one axle | £150–£360 | Only when a shock leaks or stops damping |
| Alignment afterwards | £40–£60 | Front struts only |
Independent garage, fitted. A rusty casing cannot be repaired, only replaced, so there is no halfway job to pay for. Shocks are changed as an axle pair. Your model’s price is on the shock absorber replacement cost page.
Can I still drive with this advisory?
The signs of a shock that has actually failed are different: the car bounces more than once after a bump, dives under braking, or there is oil on the shock body. None of those comes from surface rust.
Enter your reg and see every MOT your car has had. If a misting advisory has ever appeared on the same corner, you will find it there.
Which cars get this advisory most?
The all-vehicle average is 2.9 per 1,000 tests. These models see it several times as often.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Mitsubishi L200 | 22 | 7.5× | 935,407 |
| Renault Scenic | 19 | 6.6× | 600,691 |
| Renault Megane | 17 | 5.9× | 1,634,315 |
| Saab 9-3 | 14 | 4.8× | 589,429 |
| Volkswagen Amarok | 14 | 4.8× | 183,218 |
| Volkswagen Caddy | 13 | 4.5× | 1,459,681 |
| Suzuki Alto | 11 | 3.7× | 556,815 |
| Fiat Punto | 10 | 3.6× | 1,031,944 |
| Mercedes-Benz Sprinter | 10 | 3.4× | 2,101,182 |
| Volkswagen Crafter | 9 | 3.0× | 585,956 |
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.
There is no single explanation for this list. Pickups and vans are on it, led by the Mitsubishi L200 at 7.5 times the average, and their rear shocks hang low and exposed. The Renault Scenic and Renault Megane are in second and third place, and both also rank high for suspension arm ball joint play.
What the models share is rear shocks mounted out in the open. If yours is one of them, expect to see this line again.
What to do next
- Check the shock is dry. Wipe it and look again after a week. A dry, rusty shock can be left. A wet one is a different advisory.
- On a strut, have the spring seat looked at. It takes a garage a minute with the wheel off. It is the only part of the casing whose condition is a safety matter.
- Do nothing else. Painting or treating a shock body is not worth paying for. The part costs less than the labour to preserve it.
- Compare next year’s wording. The same line again means no change. A new line about oil or damping means it is time to price a pair.
Shock absorber casing damage advisory FAQ
What does shock absorbers has slight external damage to the casing mean?
The outer body of a shock absorber is rusty, dented or marked, but it is not leaking and still works. Testers use this wording mainly for corrosion. It is an advisory, so the car passes.
Is a corroded shock absorber an MOT fail?
No, not while it still works. A shock absorber fails the MOT if it is damaged to the extent that it does not function, has a serious leak, or is insecure. Surface rust on the casing is an advisory.
Will it fail next year?
Probably not. According to DVSA MOT data analysed by PriceMyRepair, 4.3% of 452,485 such advisories failed on the same shock absorber at the next test, and 23.2% were given the same advisory again.
Do I need to replace a shock absorber with a rusty casing?
Not for the rust alone. Replace it when it leaks, stops damping, or on a strut when the spring seat is corroding. Until then it can be left.
How much do new shock absorbers cost?
A pair of shock absorbers costs £150–£360 per axle fitted at an independent garage. They are always replaced in pairs. These are estimates and final quotes come from garages.
Which cars get this advisory most?
The Mitsubishi L200, at 7.5 times the average rate, then the Renault Scenic, Renault Megane and Saab 9-3. It is rare 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 1,443,236 times; 452,485 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.