Abstract: In a double air-fuel ratio sensor system including two air-fuel ratio sensors upstream and downstream of a catalyst converter provided in an exhaust gas passage, the actual air fuel ratio is adjusted in accordance with the air-fuel ratio correction amount calculated by using the output of the upstream-side air-fuel ratio sensor and the output of the downstream-side air-fuel ratio sensor. In this system, a detection whether or not the catalyst converter is deteriorated is carried out by using the output of the downstream-side air-fuel ratio sensor when the feedback control condition is satisfied. This detecting operation is prohibited when the upstream-side air-fuel ratio sensor is in an abnormal state.

Abstract: Knocking in an engine is prevented from occurring by retarding an ignition timing. A basic ignition advance angle THB and a correction value THK for retardation of the ignition timing are computed in order that the ignition timing THF is calculated by using the formula THF=THB-THK. The knocking in the engine tends to take place, as the ignition timing advances. Determination is made as to whether the correction value THK is within a reference range AR. When such a determination is that the correction value THK is within the reference range, discrimination is made as to whether an octane number of the fuel to be supplied to the engine is less than a predetermined octane number of the fuel which is optimum for the engine. If the discriminated octane number of the fuel is less than the predetermined octane number, the ignition timing is retarded by 10 degree of a rotational angle of a crank shaft.