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.
Repair cost
A gasket and clamp at a flange is a small job. A front section with a flexi, or a manifold with sheared studs, is a much bigger one because of the labour involved in freeing rusted fixings.
Before the retest
After exhaust work, start the car from cold and listen for blowing or ticking at the joints, and feel (carefully, before it gets hot) for any section that rattles or hangs low. Every original mounting rubber should be present. A sealed system matters for the emissions part of the test as well, because a leak upstream of a sensor can upset the readings.
Retest rules
If the vehicle stays at the test centre for repair and is retested within 10 working days, a partial retest is often free; some items qualify for a free retest if you bring the car back by the end of the next working day. The exact conditions are set out on GOV.UK, so confirm with the test centre before you leave. A major or dangerous failure means the car cannot legally be driven away unless the previous MOT is still valid and nothing dangerous was recorded — check your certificate first.
Related help
More exhaust failures · MOT failure decoder · Ask a mechanic about your certificate
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.