MOT defect file
Suspension mounting corroded or insecure
Last updated
Quick answer
A suspension component is loose on its mounting, or the metal around the mounting has corroded. Rust within 30cm of a suspension mounting point falls in a prescribed area, where DVSA sets the standard the repair has to meet.
- Typical cost
- £120–£700
- DIY viability
- Garage job
What you might notice
- A deep clonk under braking or over bumps, coming from the point where a component bolts to the body rather than from the joint itself
- Rust scabbing or bubbling paint on the underside around a subframe or arm mounting, often with flakes coming away in your hand
- Steering that feels as though the whole front end shifts slightly before the car responds
- Frequently nothing detectable from the driver's seat — mounting corrosion is on the underside, which is why the test result is usually the first news of it
What causes it
- Road salt and standing water at mounting points, where mud packs in behind the arm and holds moisture against the steel indefinitely
- Blocked drain holes in a sill or box section next to the mounting, so a sealed cavity slowly fills
- Fasteners left loose after earlier work, or a mounting bolt that has stretched and lost its clamping force
- Galvanic corrosion around a steel bolt in an alloy subframe, or a previous repair plated over rather than cut out
Advisory or failure?
Two separate defects share this heading and they are graded differently. A suspension component that is insecure on its mounting — a loose or missing bolt, a fractured bracket — is a major defect, and dangerous where it is likely to become detached or affects the vehicle's steering or stability. Corrosion is judged by location: any structure within 30cm of a suspension mounting point is a prescribed area, and corrosion there that has weakened the metal is a major defect, escalating to dangerous where the vehicle's structural integrity is affected. Surface rust with no loss of strength is an advisory or a minor defect. Within a prescribed area the repair must be continuously seam welded and at least as strong as the original — filler, glass fibre and riveted or bonded plates are specifically not acceptable, and neither is underseal sprayed over the top.
Can you fix it yourself?
Garage job
An insecure mounting is occasionally as simple as a bolt tightened to the correct torque with a new fastener where the manufacturer specifies a single-use one — worth checking, but do not tighten a bolt through a rubber bush with the car hanging on stands, because the bush will be preloaded and tear itself apart within months. Corrosion in a prescribed area is a different matter: it is structural welding in thin metal near fuel and brake lines, and it needs cutting back until sound steel appears. Have it assessed on a ramp before the test rather than after, and ask specifically whether the repair area falls inside 30cm of a mounting point, because that single question decides how the job has to be done.
What it costs
The two ends of this band are very different jobs. Retightening or replacing mounting bolts and bushes is £120 to £250, mostly labour. A localised welded repair to a corroded mounting area is £300 to £550 once the strip-down and paint protection are included, and a subframe mounting or chassis leg repair with the subframe dropped reaches £700 and past it. The steel is almost free; what you are paying for is access, welding time and the extra metal that appears once the rot is cut back. A new subframe on a car where the mounting is not repairable falls outside this band entirely.
What happens if you ignore it
A mounting exists to hold a suspension component against the exact loads a pothole or a hard stop puts through it, and corroded metal fails at precisely that moment. If a subframe or arm mounting tears out, the wheel changes position under the car and there is no steering correction for that. The rot also spreads outwards from inside the box section, so the visible patch is always the smallest part of the problem and every month of delay adds to the eventual bill. Because the defect is graded major or dangerous, continuing to use the car also means using a vehicle in a condition that carries a fine and penalty points.
Common questions
- Is a corroded suspension mounting an MOT failure?
- Yes where the corrosion has weakened the metal, because anything within 30cm of a suspension mounting point is a prescribed area. That is a major defect, and dangerous where structural integrity is affected. A component that is insecure on its mounting — a loose or missing bolt, a fractured bracket — is also a major defect, while surface rust with no loss of strength is an advisory.
- Why does rust within 30cm of a suspension mounting matter more than rust elsewhere?
- Because that distance defines a prescribed area, where the tester judges remaining strength rather than appearance and the repair standard is fixed. The metal has to be cut back to sound steel and replaced by continuous seam welding, at least as strong as the original. Filler, glass fibre, bonded or riveted plates and underseal are specifically not acceptable.
- What causes a deep clonk from the front under braking or over bumps?
- One common source is the mounting rather than the joint: a loose bolt or corroded metal lets the whole component shift slightly before it takes the load. It often comes with steering that feels as though the front end moves before the car responds. It needs a ramp to confirm, since mounting corrosion is on the underside and invisible from the driver's seat.
- Can a loose suspension mounting bolt simply be tightened up?
- Sometimes, using a new fastener where the manufacturer specifies a single-use one, but check the surrounding metal is still sound first. Never tighten a bolt that passes through a rubber bush with the car hanging on axle stands: the bush ends up preloaded in the wrong position and will tear itself apart within months.
- How much does a corroded suspension mounting repair cost?
- Retightening or replacing mounting bolts and bushes is £120 to £250, mostly labour. A localised welded repair to a corroded mounting area is £300 to £550 once strip-down and paint protection are included, and a subframe mounting or chassis leg repair with the subframe dropped reaches £700 and past it.
Cars this happens to most
Of every model with at least 2,000 tests in the DVSA data, these record suspension mounting corroded or insecure on the largest share of their MOTs. Across all of them the typical rate is 1.5% of tests.
- Mazda Bongo21%
1,840 times in 8,769 tests
603 times in 3,513 tests
- Suzuki Carry17%
586 times in 3,464 tests
2,770 times in 19,214 tests
- Toyota Hiace14%
849 times in 6,129 tests
- Rover 7512%
727 times in 6,210 tests
- Honda Fr-V12%
686 times in 5,962 tests
554 times in 4,826 tests
A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.
Warning lights behind this
This defect is what a tester writes down. What you see from the driver’s seat is a symbol, and it is worth checking which one, because manufacturers reuse the same shape for more than one system.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.