MOT defect file
Exhaust leaking or insecure
Last updated
Quick answer
Gases are escaping before the tailpipe, or a section is loose. Can be a clamp and a joint, or a full section.
- Typical cost
- £60–£350
- DIY viability
- Possible with care
What you might notice
- A deeper, harsher note than usual, loudest at idle and on a cold start
- Blowing or puffing that rises and falls with the throttle
- A rattle or drone at particular engine speeds, which usually means a broken mounting rubber or a loose heat shield
- The smell of exhaust inside the car, especially with the ventilation set to recirculate
What causes it
- Corrosion at welded joints or along the underside of a silencer, where condensation collects on short journeys
- A failed manifold or downpipe gasket, or a cracked manifold flange
- Perished rubber hangers letting a section drop and stressing the joints either side of it
- A snapped manifold stud or a corroded clamp at a section joint
Advisory or failure?
An exhaust that is leaking or insecure is a major defect and a straight failure — there is no advisory route for an actual leak, though a section that is visibly corroded but still gas-tight is commonly recorded as an advisory. A system producing noise appreciably greater than a comparable vehicle with a standard system in good order also fails as a major defect. Separately, a missing catalytic converter, or a missing diesel particulate filter where one was fitted from the factory, is a major defect in its own right — and where there is evidence that a DPF has been cut out of the system, the tester must refuse to carry out the test at all. A section hanging loose enough to fall onto the road is graded dangerous.
Can you fix it yourself?
Possible with care
Replacing a rear box or a clamp on a car that has lived somewhere dry is achievable with axle stands, penetrating oil and patience. The realities are that exhaust fasteners are the most corroded on the vehicle, you will very likely end up cutting one off, and you are working underneath a car among hot, sharp components. Manifold work is a different proposition — snapped studs in an alloy cylinder head can escalate badly. Exhaust paste and bandage will quieten a small blow for a while, but it is not a repair and a tester will fail a bandaged joint that is still leaking.
What it costs
A clamp, a hanger or a gasket is £10 to £30 in parts with under an hour of labour, which is the bottom of the band. A rear silencer section is £50 to £150 fitted and a centre section with a flexi-joint £120 to £250. Costs climb when the fastenings are seized, when the system is a one-piece assembly that cannot be part-replaced, and fastest of all when the fault is at the manifold. Anything involving the catalytic converter or a DPF is a different order of expense and sits outside this band.
What happens if you ignore it
The immediate concern is carbon monoxide: a leak ahead of the passenger compartment can draw fumes into the cabin, and carbon monoxide gives no warning before it makes you drowsy. Mechanically, a leak upstream of the lambda sensor confuses the fuelling and can push the engine into a rich running condition that poisons the catalyst, turning a £60 gasket into a £500 catalytic converter. A section that finally parts company at speed can also damage the underside of the car and become a hazard for the vehicle behind.
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.