AIR/FUEL CONTROL SYSTEM PROVIDING CATALYTIC MONITORING

A control system which provides a measurement of efficiency in a catalytic converter coupled to the engine exhaust. For each inducted airflow range in which the engine is operating, test periods are generated by counting transitions in the output of an upstream exhaust gas oxygen sensor. These test periods are limited to a maximum count. Concurrently, during each test period, transitions are counted in a downstream exhaust gas oxygen sensor. Upstream and downstream transitions are accumulated and a ratio of the accumulated totals determines converter efficiency.