Typical engine emission systems maintain engine air/fuel operation within the peak efficiency window of a catalytic converter. Generally, engine air/fuel ratio is adjusted in response to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter such as shown in U.S. Pat. No. 5,115,639. It is also known to monitor the performance of the converter by comparing a ratio in transitions of the downstream sensor output to the upstream sensor output.
The inventors herein have recognized numerous problems with the above approaches when exhaust gases are routed through an auxiliary converter of lower mass via a bypass solenoid valve. These systems are used to more rapidly warm the auxiliary converter thereby achieving more rapid catalytic action during cold engine starts. Prior performance monitoring systems could not provide complete diagnostic monitoring of the auxiliary converter and bypass valve. Nor could such systems determine whether degraded operation resulted from the converters or from the bypass valve.