ABNORMALITY DIAGNOSIS SYSTEM OF DOWNSTREAM SIDE AIR-FUEL RATIO DETECTION DEVICE

The abnormality diagnosis system of a downstream side air-fuel ratio detection device , comprises an air-fuel ratio control part controlling an air-fuel ratio of an air-fuel mixture, an abnormality judgment part judging abnormality of the downstream side air-fuel ratio detection device based on a characteristic of change of output of the downstream side air-fuel ratio detection device when the air-fuel ratio control part makes the air-fuel ratio of the air-fuel mixture change, and an oxygen change calculation part calculating an amount of change of an oxygen storage amount of the catalyst when the air-fuel ratio control part makes the air-fuel ratio of the air-fuel mixture change. The abnormality judgment part does not judge abnormality of the downstream side air-fuel ratio detection device when the amount of change of the oxygen storage amount is less than a lower limit threshold value.