DETERIORATION DIAGNOSIS APPARATUS FOR AN EXHAUST GAS PURIFICATION APPARATUS

Diagnosing deterioration of an SCR catalyst (5) using air-fuel ratio sensors (7, 8). A deterioration diagnosis apparatus for an exhaust gas purification apparatus performs an induction process wherein reducing agent is supplied to a hydrogen production catalyst (4), taking account of the degree of deterioration of the hydrogen production catalyst, when an internal combustion engine is operating at a lean air-fuel ratio, measures the air-fuel ratio of the exhaust gas flowing into the SCR catalyst (5) and the air-fuel ratio of the exhaust gas flowing out of the SCR catalyst (5) by the air-fuel ratio sensors during the period through which the induction process is performed, and diagnoses deterioration of the SCR catalyst based on the difference between the air-fuel ratios thus measured.