Patent ID: 7168237

Claim:
A catalyst deterioration diagnosing device comprising: an exhaust gas purification catalyst provided in an exhaust passage of an engine; an upstream side sensor which detects a characteristic value of an exhaust gas on an upstream side of the catalyst; a downstream side sensor which detects a characteristic value of the exhaust gas on a downstream side of the catalyst; and a controller into which a detection signal of the sensors is input, functioning to: determine the engine air-fuel ratio operating condition; when the engine air-fuel ratio operating condition is determined to be a non-stoichiometric air-fuel ratio operating condition, determine whether the catalyst has deteriorated on the basis of a value expressing a difference in an oxygen concentration on the upstream side and an oxygen concentration on the downstream side of the catalyst, determined from the two detected characteristic values; and when the engine air-fuel ratio operating condition is determined to be a non-stoichiometric air-fuel ratio operating condition, and the catalyst is determined to have deteriorated on the basis of the value expressing the difference in the oxygen concentration, feedback-control an air-fuel ratio of the engine to a stoichiometric air-fuel ratio, and determine again whether the catalyst has deteriorated on the basis of a value expressing a difference in an air-fuel ratio variation on the upstream side and the downstream side of the catalyst, determined from the two detected characteristic values.