What is a canister purge valve

A Canister Purge Valve is generally opened when the vehicle's ECM (Engine Control Module) applies a voltage to the valve. This valve controls how much (and when) fuel vapor is allowed to leave the fuel tank and be re-injected into the throttle body. The vehicle's ECM measures many factors (O2 levels, Exaust, Coolant temperature, etc.) to determine how much of this vapor can be re-injected, To ensure proper air/fuel mixing in the manifold. The purpose of the (Carbon) canister is to allow storage of fuel vapors when the engine is not running. Fuel vapors tend to stick to the carbon pellets in the canister, which is then picked up and used as soon as the engine is started.