ABNORMALITY DIAGNOSIS SYSTEM OF AIR-FUEL RATIO SENSOR

摘要

An abnormality diagnosis system which can judge a downstream side air-fuel ratio sensor to be normal accurately is provided. The abnormality diagnosis system of an air-fuel ratio sensor is provided with an exhaust purification catalyst, an upstream side air-fuel ratio sensor which is provided at an upstream side of the exhaust purification catalyst, a downstream side air-fuel ratio sensor which is provided at a downstream side of the exhaust purification catalyst, and a diagnosis device which uses the outputs of these air-fuel ratio sensors as the basis to diagnose abnormality of the downstream side air-fuel ratio sensor. The diagnosis device judges that the downstream side air-fuel ratio sensor is normal when a time counter which counts the time when the output air-fuel ratio of the upstream side air-fuel ratio sensor becomes a rich air-fuel ratio which is richer than the stoichiometric air-fuel ratio and the output air-fuel ratio of the downstream side air-fuel ratio sensor becomes a rich judgment reference air-fuel ratio which is richer than the stoichiometric air-fuel ratio or less becomes a predetermined judgment value which is larger than zero or more.