Abstract: In an air-fuel ratio control apparatus for controlling the air-fuel ratio of an engine in accordance with the output of an O.sub.2 sensor, in response to the starting of the engine the output voltage of the O.sub.2 sensor is sampled at intervals of a unit time. The slope of the output voltage waveform of the O.sub.2 sensor is computed from the successively sampled values. The computed value is compared with a predetermined value indicative of a slope value attained when the O.sub.2 sensor is activated and thus it is considered that the O.sub.2 sensor is activiated when the computed value is greater than the predetermined value.

Abstract: The output of a knock sensor for detecting a knock signal of an engine is applied to a bandpass filter for bandpassing only the knock signal. The output of the bandpass filter is applied to an automatic gain control circuit for variably controlling the gain of the knock signal. The output of the automatic gain control circuit is applied to an averaging circuit via a maximum clamp circuit. A comparator supplies an output to an integration circuit of the next stage when the output of the bandpass filter becomes greater than that of the averaging circuit. An ignition control device for controlling the spark timing of an ignition coil energizes the ignition coil by the output from a pick-up coil and that of the integration circuit and delivers a feedback signal to the integration circuit.