Leak Detection Pump (EVAP Leak Check)

On many cars the test runs automatically after the engine is switched off. It can detect leaks as small as about half a millimetre in diameter. A loose or faulty fuel cap is a common reason for a leak code. Some cars use a natural vacuum test instead.

The pump changes the pressure in the sealed system, and the control unit measures how quickly it returns to normal.

Tighten or replace the fuel cap before more complex diagnosis. A smoke test is the usual way to find the actual leak.