What is the function of a canister purge valve?

The function of a vehicle's canister purge valve is to control exactly how much fuel vapor gets purged from the charcoal canister where the vapors are temporarily stored after being trapped by the Evaporative Emission Control (EVAP) system. The purpose of this entire system is to keep potentially harmful fuel vapors from leaking into the air.

The purge valve is inactive when the engine is shut off. Once a vehicle is running, the purge valve gradually opens, as instructed by the vehicle's engine computer, to allow restricted amounts of fuel vapor to be accessed and burned up in the engine.