Method and apparatus for monitoring catalyst efficiency and secondary air injection

Method and apparatus to monitor secondary air injection and catalyst conversion efficiency. The method includes operating an engine in a rich condition after detecting an engine steady state condition. The secondary air injector injects air into an exhaust stream to simulate a lean engine condition. The injection of the air into the exhaust stream is ceased after both inlet and outlet sensors detect the lean condition. After ceasing air injection, a lag time is determined between the inlet sensor detecting the rich condition and the outlet sensor detecting the rich operating condition. An oxygen storage capacity of the catalytic converter is calculated based on the lag time. An efficiency of the catalytic converter is determined as a function of the storage capacity. Additionally, performance of the secondary air injector is monitored. If the inlet sensor fails to detect the lean condition after the secondary air injector is active, a fault is signaled.