Patent ID: 8205435

Claim:
A deterioration determination device for determining deterioration of a catalyst that is provided in an exhaust passage of an internal combustion engine, and has an oxygen storage capability for storing oxygen in exhaust gases exhausted from the engine when the exhaust gases form an oxidation atmosphere, the catalyst releasing stored oxygen when the exhaust gases form a reduction atmosphere, and reducing exhaust emissions, comprising: an upstream oxygen concentration parameter sensor that detects an upstream oxygen concentration parameter indicative of an oxygen concentration in exhaust gases on an upstream side of the catalyst in the exhaust passage; a downstream oxygen concentration parameter sensor that detects a downstream oxygen concentration parameter indicative of an oxygen concentration in the exhaust gases on a downstream side of the catalyst in the exhaust passage; control means configured to control the exhaust gases flowing through said upstream oxygen concentration parameter sensor into the catalyst, between the oxidation atmosphere and the reduction atmosphere; and deterioration determining means configured to determine a deterioration of the catalyst based on a result of comparison between the upstream oxygen concentration parameter for comparison which is detected when a first predetermined time period elapses after the upstream oxygen concentration parameter exceeds a first predetermined value, and the downstream oxygen concentration parameter for comparison which is detected when a second predetermined time period, which is equal to the first predetermined time period, elapses after the downstream oxygen concentration parameter exceeds a second predetermined value, which is equal to the first predetermined value, after said control means switches the exhaust gases from the oxidation atmosphere to the reduction atmosphere.