AIR-FUEL RATIO CONTROLLER OF INTERNAL COMBUSTION ENGINE

PURPOSE:To avoid deterioration of exhaust efficiency while securing good operability by controlling carrying-out of air-fuel ratio control according to the deterioration degree even if an exhaust gas concentration sensor is deteriorated. CONSTITUTION:The deterioration degree of an O2 sensor is judged on the basis of a reversal rotational cycle tCYCLE of the O2 sensor 19 by dividing the deterioration degree into normal, high, and low. When the O2 sensor 19 is normal, carrying-out of airfuel ratio control is permitted, and an alarm lamp 20 is turned off. On the other hand, when it is judged that the O2 sensor 19 is deteriorated, whether the deterioration is low or not is judged. When the deterioration degree is high, carrying-out of the air-fuel ratio control is prohibited, and an ON command is issued to the alarm lamp 20. Moreover, when the deterioration degree is low, carrying-out of the air-fuel ratio control is permitted, and an ON command is issued to the alarm lamp 20.