Exhaust MOT advisories: minor leaks, corrosion and loose mounts
Exhaust MOT advisories mean part of the exhaust is rusting, blowing slightly at a joint, or hanging off a tired rubber mount, but not badly enough to fail. Most are cheap to fix if you catch them early. A small leak tends to get louder and bigger, and once it's affecting emissions or the noise level, it's a fail.
The common exhaust advisories
| Advisory wording | What it usually means | Rough repair cost |
|---|---|---|
| Exhaust has a minor leak of exhaust gases | A blow at a joint, a gasket or a small hole, often near the back box | £40 to £200 |
| Exhaust corroded | Surface rust on pipes or boxes, not yet holed | Watch it |
| Exhaust rubber mounting deteriorated | A rubber hanger is cracked or stretched | £5 to £40 |
| Exhaust insecure | Something's loose or a bracket has rusted off | £30 to £120 |
The costs are typical independent garage prices. A full rear silencer usually runs £80 to £200 fitted on a normal car. A catalytic converter is a different league, often several hundred pounds or more.
When the exhaust fails the MOT
The relevant rules are in section 8 of the MOT manual:
- An exhaust or induction leak that could affect emissions is a major defect.
- Exhaust noise above what's permitted is major.
- Any part of the silencer system that's insecure is major, and one that's likely to fall off is dangerous.
Leaks before the catalytic converter or lambda sensor are the ones most likely to upset the emissions test, because they let air in and confuse the sensors.
Smoke and diesel particulate filters
On diesels, the MOT is strict about smoke. A diesel with a DPF (diesel particulate filter) fails as a major defect if it puffs out visible smoke of any colour during the test. Evidence that the DPF has been tampered with, such as a filter that's been cut open and welded back without a good reason, is also a major fault. Removing a DPF isn't a way to save money. It will fail.
If a diesel won't pass the smoke test, the cause is often a blocked DPF, a sticking EGR valve, a boost leak or injector trouble. Those need diagnosing before anyone starts replacing parts.
On petrol cars, dense blue or black smoke at idle or under acceleration is also a major fault, as is an engine warning light that's on.
Why leaks are worth fixing early
A blow at a joint usually starts as a tick or a puff you can only hear with the window down. Over a few months the hot gas erodes the gap and it gets louder. Then there's the safety side: a leak under the cabin can let exhaust fumes, including carbon monoxide, into the car, especially when you're sitting in traffic with the heater on recirculate.
Rubber mounts are the cheapest part of the whole system. Leaving a broken one lets the exhaust swing and knock, which cracks joints and brackets.
Can you DIY it?
Replacing rubber hangers is a five-minute job with the car safely raised. A rear box that clamps on is manageable at home too, though old clamps and joints are usually seized solid. Anything near the catalytic converter or the manifold, or that needs welding, is better left to an exhaust specialist.
What to do now
Have a look underneath with the engine running (and the car out of gear, handbrake on). You'll often hear exactly where a leak is, and you can feel the puffs by holding a hand near, not on, the pipe. Then get a quote from an exhaust centre or garage.
If there was an oil leak advisory too, oil dripping onto the exhaust can cause smoke and smells, so read the oil leak guide. And for your full result, use the MOT advisory decoder.
Got more than one advisory?
Paste your whole MOT result into the decoder. It explains every line, sorts them by urgency and adds up a rough repair budget.
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