Abstract: An air/fuel control system (10) and method for an engine (28) coupled to a catalytic converter (50) includes monitoring of the converter (50). Engine air/fuel ratio is controlled (104-178) in response to a pair of two-state signals generated by comparing the outputs of respective upstream (44, 55) and downstream (52) exhaust gas oxygen sensors to a reference voltage. The transitions in the downstream sensor are indicated when a difference in its output amplitude changes signs and a difference between stored peak values occurring during two successive changes in the sign exceeds a predetermined value (200-350). Converter efficiency is indicated from a ratio of transitions in the downstream sensor transitions to transitions in the upstream sensor output which are generated in a similar manner (690-770).