Abnormality diagnosis system of downstream side air-fuel ratio detection device

An abnormality diagnosis system of a downstream side air-fuel ratio detection device arranged in an exhaust passage of the internal combustion engine at a downstream side of a catalyst, comprising:

• an air-fuel ratio control part configured to control an air-fuel ratio of an air-fuel mixture in a combustion chamber of the internal combustion engine;
• an abnormality judgment part configured to judge 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 configured to calculate 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, wherein
• the abnormality judgment part is configured not to 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.