The vehicle evaporative emission control system purge valve regulates how much fuel vapor the system purges from the charcoal canister that traps and stores the vapor. When the engine is running, sensors trigger the check engine light if the purge valve allows a vapor flow greater or less than normal.
The purge valve connects to the vehicle's engine, the charcoal canister that is part of the EVAP, and the engine's computer. If the purge valve does not close completely, or if it sticks, the check engine light may illuminate. It is common for the valve to stick immediately after pumping gas into a vehicle, and the car may not start, or it may run rough initially. It is usually easy to test and replace a purge valve.