Air/fuel ratio controller for internal combustion engine

This invention relates to a device for detecting and correcting fluctuations in the output characteristics of an air/fuel ratio sensor for an internal combustion engine. A catalyst which has an oxygen storage function is provided in the exhaust gas pipe of an engine The oxygen storage amount is estimated based on the output of the air/fuel ratio sensor upstream of the catalyst The air/fuel ratio is controlled so that the oxygen storage amount coincides with the target value. When the output of the downstream air/fuel ratio sensor continuously displays a rich or a lean value for more than a determination time which is varied continuously with respect to operating conditions, deterioration in the upstream air/fuel ratio sensor is detected. Thus the output of the upstream air/fuel ratio sensor is corrected based on the output of the downstream air/fuel ratio sensor In this manner, output fluctuations are corrected based on the deterioration of the air/fuel ratio sensor upstream of the catalyst.