The MOT Files

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
Exhaust system with a leaking jointAn exhaust is shown in side elevation from engine manifold to catalytic converter, a leaking pipe joint, rear silencer and tailpipe. Red jets rise from a gap at the joint to show escaping exhaust gas before it reaches the silencer. A leak that allows gases to escape, or a system that no longer reduces exhaust noise effectively, is an MOT failure.Catalytic converterGas escaping at jointRear silencerManifoldTailpipeLeak before the silencer — fail
The exhaust must be secure, free from serious leaks and effective at silencing the engine. Gas escaping at a joint, or excessive noise because the system no longer silences properly, fails the MOT.

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, as is evidence that a DPF has been tampered with. Where a filter canister has clearly been cut open and rewelded, the tester should reject it unless you can show there was a legitimate reason for the cut, such as professional off-car cleaning. 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.

Common questions

Does an exhaust leak fail an MOT?
Yes. An exhaust that is leaking or insecure is a major defect and a straight failure, with no advisory route for an actual leak, although a section that is visibly corroded but still gas-tight is commonly recorded as an advisory. A system appreciably louder than a standard one in good order also fails, and a section hanging loose enough to fall onto the road is graded dangerous.
Is removing a DPF an MOT failure?
Yes, and it fails as a major defect. A missing catalytic converter, or a missing diesel particulate filter where one was fitted from the factory, is a major defect, as is evidence that a DPF has been tampered with. Where a filter canister has clearly been cut open and rewelded it will be failed unless you can show there was a legitimate reason, such as professional off-car cleaning.
Is it safe to drive with a blowing exhaust?
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. A leak upstream of the lambda sensor also confuses the fuelling and can push the engine into a rich condition that poisons the catalyst, turning a £60 gasket into a £500 converter.
Will exhaust paste or bandage get me through the MOT?
No. 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. Treat it as a stopgap to get the car to the garage rather than as a way of passing the test.
How much does it cost to fix an exhaust leak?
A clamp, a hanger or a gasket is £10 to £30 in parts with under an hour of labour. 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 one piece and cannot be part-replaced, and fastest of all when the fault is at the manifold.

Cars this happens to most

Of every model with at least 2,000 tests in the DVSA data, these record exhaust leaking or insecure on the largest share of their MOTs. Across all of them the typical rate is 1.2% of tests.

  1. Fiat Bravo7.1%

    270 times in 3,778 tests

  2. Hyundai Coupe7.0%

    342 times in 4,861 tests

  3. Fiat Grande Punto6.9%

    364 times in 5,271 tests

  4. Volkswagen Fox6.7%

    1,552 times in 23,316 tests

  5. Talbot Express5.9%

    283 times in 4,826 tests

  6. Peugeot 2065.5%

    1,873 times in 34,316 tests

  7. Toyota Previa5.2%

    200 times in 3,852 tests

  8. Volvo 900 Series4.9%

    143 times in 2,898 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.

Pass rates for every make and model

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.

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