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Is an Exhaust Cutout Legal in the UK?

Not legal

An exhaust cutout is not road-legal when it is open. Opening the valve routes the gases around the silencer, and UK law requires the silencer to stay effective and the car not to be made louder than its type-approved level.

Why?

A cutout is a valve in the exhaust that, when opened, lets the gas exit before the silencer for maximum noise. That is exactly what the rules prohibit: Regulation 54 says a vehicle must be fitted with a silencer and must not have its exhaust altered so it makes more noise than the level it was type-approved at. Running with the cutout open defeats the silencer, so it breaks the rule the moment you open it on the road.

At the MOT, an exhaust that is clearly louder than standard fails the nuisance check, and at the roadside an obviously bypassed silencer is an easy offence to spot and prove. The excessive-noise regulation backs this up separately.

Kept permanently closed, a cutout is just extra pipework, but the whole point of fitting one is to open it, and open on the road it is not legal. It is a track or off-road part.

What decides if it's legal

  • The exhaust silencer must stay effective at all times on the road.
  • A cutout opened on the road bypasses the silencer, which is not allowed.
  • The exhaust must not be altered to increase noise beyond the type-approved level.
  • Open-cutout use fails the MOT nuisance check and is a roadside offence.

Does it depend on your car?

The silencer and noise requirements apply to every car. Opening a cutout bypasses the silencer on any of them, so the verdict does not change from one vehicle to another.

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Sources

This page is general guidance, not legal advice, on the rules in the UK for exhaust cutout. The detail varies by exact vehicle and changes over time - confirm with your insurer and the latest DVSA and GOV.UK guidance before modifying.