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Is an Oil Catch Can Legal in the UK?

Conditional

An oil catch can is legal in the UK when the crankcase ventilation system stays sealed and recirculating. Vent-to-atmosphere setups are not road-legal.

Why?

A catch can sits in the breather line between the crankcase and the intake, trapping oil mist that would otherwise coat the intake and valves. Fitted in-line with the system still sealed - gases still routed back into the engine - it leaves the car's emissions control exactly as designed, and there is nothing illegal about it.

The illegal version is the same hardware plumbed the wrong way: a breather or "vented" can that releases crankcase fumes to the air. Crankcase emissions are part of the car's type approval, so venting them breaches the emissions regulations and can surface at the MOT.

Beyond that it is maintenance, not law: the can needs emptying regularly, and like any modification it should be declared to your insurer.

What decides if it's legal

  • A catch can plumbed back into a sealed crankcase ventilation system is legal.
  • Venting the crankcase to atmosphere (a breather that dumps fumes) breaches emissions rules.
  • The can must be emptied regularly to keep working.
  • Declare the modification to your insurer.

Does it depend on your car?

Direct-injection turbo engines benefit most, which is why catch cans cluster around them. The legality is identical everywhere; your car's page shows kits with the right fittings for your engine.

Related UK legality guides

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Sources

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