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What is a purge valve and charcoal canister?

A purge valve is the part of the vehicle Evaporative Emission Control (EVAP) system.

The EVAP system prevents fuel vapours in the fuel tank from escaping into the atmosphere. The EVAP system traps fuel vapours from the fuel tank and temporarily stores them in the charcoal canister.

When the engine is running under certain conditions, the fuel vapours are purged from the canister and burned inside the engine.

The purge valve precisely controls the amount of fuel vapour that is purged from the charcoal canister.

In modern cars, the purge valve is an electrically-operated solenoid that is controlled by the engine computer (Engine Control Unit – ECU).

When the engine is off, the purge valve is closed. When the engine is running and fully warmed up, the engine computer gradually opens the purge valve to allow some amount of fuel vapour to be moved from the charcoal canister to be burned in the engine. The purge flow is monitored by a number of sensors. If the purge flow is less or more than is expected under the conditions, the computer illuminates the “Check Engine” light.