Gas is escaping from somewhere other than the tailpipe. As well as being noisy, a leak ahead of the sensors can upset the fuelling, and a leak under the floor can let fumes into the cabin.
Wording as it commonly appears on an MOT certificate: “Exhaust has a major leak of exhaust gases”.
What does it mean?
Gas is escaping from somewhere other than the tailpipe. As well as being noisy, a leak ahead of the sensors can upset the fuelling, and a leak under the floor can let fumes into the cabin.
How serious is it?
Usually recorded as: Major. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- A blown gasket at a flange joint
- A rusted-through section, usually at the back box or a bend where water sits
- A cracked manifold or a failed flexi pipe on the front section
- Broken studs at the manifold-to-downpipe joint
What a mechanic would check
Engine running, and I'll feel round the joints with a gloved hand for escaping gas, working from the manifold back. Partially blocking the tailpipe pressurises the system and makes a small leak much easier to find.
What might need replacing
- A gasket and clamp
- A section of the exhaust
- A flexi pipe or downpipe
- Manifold studs, which often shear and need drilling out
Can you still drive it?
It's a failure. The bigger concern is fumes: if you can smell exhaust in the cabin, don't drive it with the windows shut — carbon monoxide is not something to gamble with.
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This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.