An air-fuel ratio sensor malfunction detecting apparatus detects the output signal periods of a first or upstream air-fuel ratio sensor, and compares them with a predetermined value to judge the first sensor to be malfunctioning when the periods are larger than the predetermined value. The apparatus also compares the magnitudes of air-fuel ratio control quantities from a second or downstream air-fuel ratio sensor with a predetermined value to judge the first sensor to be malfunctioning when the air-fuel ratio control quantities are larger than the predetermined value.
Air-fuel ratio sensor trouble detecting apparatus