EVAP System

Petrol evaporates constantly, especially in warm weather. The evaporative emission control system routes vapour from the tank to a canister full of activated charcoal, which stores it. When conditions are right, the purge valve opens and engine vacuum draws the stored vapour into the intake to be burnt. OBD-II tests the system for leaks, which is why a loose fuel cap can switch on the engine warning light.

The system is sealed and is checked by the OBD system for leaks. Leak tests usually run when the car has been standing and the tank is within a certain fill range.

The control unit seals the system with a vent valve and checks whether vacuum or pressure holds, or uses a dedicated leak-detection pump. Leaks down to a very small size are detected. Main parts are the fuel tank, the charcoal canister, the purge valve, a vent valve and often a pump or pressure sensor used for the leak test.

Codes like P0455, P0442 and P0456 point to leaks; P0496 to a purge valve that lets vapour in when it should not. Most EVAP faults cause a fuel smell or a warning light rather than driveability problems. EVAP faults rarely affect how the car drives, but they cause emission failures and a lit check engine light. A loose or worn fuel cap is the cheapest and most common cause of a large-leak code.