Abnormality diagnosis system of internal combustion engine

An abnormality diagnosis system of an internal combustion engine, which internal combustion engine comprises an exhaust purification catalyst which is arranged in an exhaust passage of the internal combustion engine and which can store oxygen, a controller for controlling the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst to become a target air-fuel ratio, and a downstream side air-fuel ratio sensor which is arranged at a downstream side of said exhaust purification catalyst in an exhaust flow direction and which can detect an air-fuel ratio of exhaust gas discharged from said exhaust purification catalyst,

• wherein the abnormality diagnosis system of an internal combustion engine performs an active air-fuel ratio control which alternately controls said target air-fuel ratio to a rich air-fuel ratio which is richer than a stoichiometric air-fuel ratio and a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio at a given interval, and diagnoses abnormality of said exhaust purification catalyst based on the output air-fuel ratio of the downstream side air-fuel ratio sensor during execution of said active air-fuel ratio control, and
• wherein said abnormality diagnosis system judges that said exhaust purification catalyst is abnormal when the output air-fuel ratio of the downstream side air-fuel ratio sensor reaches a rich judgment air-fuel ratio which is richer than the stoichiometric air-fuel ratio or less and a lean judgment air-fuel ratio which is leaner than the stoichiometric air-fuel ratio or more in the period where said target air-fuel ratio is successively made the rich air-fuel ratio and lean air-fuel ratio due to said active air-fuel ratio control, and
• judges that said air-fuel ratio control by said controller is abnormal when the output air-fuel ratio of the downstream side air-fuel ratio sensor reaches only one of said rich judgment air-fuel ratio or less or said lean judgment air-fuel ratio or more in the period where said target air-fuel ratio is successively made the rich air-fuel ratio and lean air-fuel ratio due to said active air-fuel ratio control.