As fuel is pumped into the tank, fuel vapors are displaced. These vapors are trapped by the on board refueling vapor recovery system before they can escape into the atmosphere via the fuel filler neck. Diagnosis and testing of this system requires special procedures.

Subaru vehicles are equipped with either a Conventional or Enhanced Evaporative Emissions Control System. Each system uses a different method to prevent unburned hydrocarbons from escaping to the atmosphere.