MOT defect file
Suspension arm damaged, distorted or corroded
Last updated
Quick answer
A suspension arm is bent, cracked or rusted beyond what the tester will accept. Arms set where the wheel sits, so a distorted one throws the alignment out and cannot be straightened — it is replaced.
- Typical cost
- £150–£340
- DIY viability
- Possible with care
What you might notice
- The steering wheel off-centre when the car is running straight, or a persistent pull to one side
- Rapid wear along one edge of a front tyre, feathered to the touch when you run a hand across the tread
- A knock from one corner over bumps, and on a badly distorted arm a wheel that sits visibly out of line in the arch
- Nothing you would notice after a light kerb strike — an arm can be bent by a few millimetres, which is enough to fail and is only found on a ramp or an alignment rig
What causes it
- A pothole or kerb impact bending the arm, which is why it is so often one side only and the alignment reading gives it away
- Corrosion eating into a pressed-steel arm, particularly along the folded seam where salt and mud collect
- A previous heavy impact repaired without replacing the arm, or an arm straightened rather than renewed
- Grounding on a raised kerb or a speed bump, which loads the arm in a direction it was never designed for
Advisory or failure?
A suspension arm that is fractured, distorted or deformed is a major defect and a failure, as is one that is insecure on the vehicle. Corrosion is graded on strength rather than appearance: an arm excessively corroded so that it is weakened is a major defect, while surface rust that has not reduced its strength is an advisory or minor defect. It becomes dangerous where the arm is damaged badly enough that it could become detached, or where the damage affects the vehicle's steering or stability — a cracked arm counts here, because a fatigue crack in a loaded component gives no warning before it lets go. Note that a bent arm is a failure even where the car drives straight, since the tester is judging the component rather than the symptom, and a repair by welding or straightening is not acceptable on a safety-critical suspension arm.
Can you fix it yourself?
Possible with care
Fitting a complete replacement arm is a realistic driveway job: axle stands rather than a jack, a torque wrench, a ball joint splitter for the taper, and penetrating oil applied the night before on the subframe bolts. Two traps catch people out. Bolts passing through rubber bushes must be finally tightened with the car at normal ride height, not hanging on the stands, or the bush is permanently twisted and will fail within months. And an arm often carries the camber adjustment, so the four-wheel alignment afterwards is part of the job rather than an optional extra. If a subframe bolt shears — common on a car that has seen a decade of salt — stop, because extracting it is beyond most home setups.
What it costs
A complete lower arm with bushes and ball joint already fitted is £45 to £120 in parts for a common car, and around an hour and a half of labour a side, so labour is the larger share of the bill. Alignment at £40 to £70 afterwards is not optional. You reach the top of the band where the arm is only supplied as part of a larger assembly, where seized or sheared subframe bolts add an hour of extraction, or where the impact that bent the arm also took out the track rod end and the wheel, which is common enough to be worth having checked at the same time.
What happens if you ignore it
A distorted arm holds the wheel out of its designed position, so the car brakes unevenly, steers imprecisely and scrubs a front tyre out in a few thousand miles — the tyre bill alone usually exceeds the repair within a year. A cracked or heavily corroded arm is a different risk: it is carrying cornering and braking loads with less material than it was designed with, and when it finally separates the wheel folds under the car. Since the defect is graded major and can be dangerous, driving on it is also using the vehicle in a condition that carries a fine and penalty points.
Common questions
- Is a bent suspension arm an MOT failure?
- Yes. An arm that is fractured, distorted or deformed is a major defect, as is one that is insecure on the vehicle. It becomes dangerous where the arm could become detached or the damage affects steering or stability. Corrosion is graded on strength: excessively corroded and weakened is major, while surface rust that has not reduced strength is an advisory or minor defect.
- Why has my car failed for a bent suspension arm when it drives straight?
- The tester is judging the component rather than the symptom, so a distortion of a few millimetres fails even when nothing is obvious from the driver's seat. Alignment set around a bent arm can also hide it for a while. Straightening or welding a safety-critical suspension arm is not acceptable either, so it is replaced.
- Why is one front tyre wearing out along its inner edge?
- A bent or displaced arm holds the wheel outside its designed position, so the tyre is dragged sideways as it rolls and wears along one edge. Run a hand across the tread and the wear feels feathered into fine ramps. It can scrub a tyre out in a few thousand miles, so the tyre bill often exceeds the repair.
- Can I change a wishbone myself?
- It is a realistic driveway job with axle stands, a torque wrench and a ball joint splitter, plus penetrating oil on the subframe bolts the night before. Two traps catch people out: bolts through rubber bushes must be finally tightened at normal ride height, and the arm often carries the camber adjustment, so alignment afterwards is part of the job. Stop if a subframe bolt shears.
- How much does it cost to replace a suspension arm?
- Between £150 and £340. A complete arm with bushes and ball joint already fitted is £45 to £120 for a common car, with around an hour and a half of labour a side, and four-wheel alignment at £40 to £70 afterwards is not optional. Seized or sheared subframe bolts push the bill towards the top of the band.
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.