Check Valves
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
The check valves prevent back flow of the exhaust gases into the secondary air injection system. An inoperative AIR pump that had shown indications of exhaust gases in the outlet port would indicate a check valve failure.