ABNORMALITY DIAGNOSIS APPARATUS OF EXHAUST GAS PURIFICATION APPARATUS

An abnormality diagnosis apparatus includes an exhaust gas purification apparatus arranged in an exhaust passage of an internal combustion engine and including a SCR catalyst; a supply apparatus supplying an additive such as ammonia to the exhaust gas purification apparatus; an EGR apparatus recirculating a part of exhaust gas from the exhaust passage at a downstream side of a position of supplying the additive to an intake passage; obtaining means for obtaining a NO x inflow amount into the exhaust gas purification apparatus; diagnosing means for calculating a physical quantity correlated to a NO x purification performance of the exhaust gas purification apparatus using the obtained NO x inflow amount, and diagnosing that an abnormality has occurred in the exhaust gas purification apparatus when the physical quantity is smaller than a threshold; and correcting means for, when the additive is recirculated together with a part of the exhaust gas, correcting the threshold to a smaller value than when otherwise.