US Patent 7131322 Vehicle evaporative system diagnostic

A diagnostic method for an evaporative emission control system of an automotive vehicle determines functionality of one of a purge valve and/or a vacuum switch and valve assembly. The vacuum switch and valve assembly is located between a carbon canister and atmosphere and is either open or closed according to whether the system is in a high or low negative pressure condition. A controller determines if the vacuum switch and valve assembly is functioning properly according to the high or low negative pressure condition. The controller determines if the purge valve is functioning properly based on engine performance characteristics.