Importance

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Abstract

A primary deterioration diagnosis of a front catalyst (8) is performed based on the air-fuel ratio of the exhaust gas downstream of the front catalyst (8) detected in a first period each time the first period elapses when the engine (1) is running in a diagnosis region. A secondary deterioration diagnosis of the front catalyst (8) is performed based on the air-fuel ratio of the exhaust gas downstream of the front catalyst (8) detected in a second period longer than the first period each time the second period elapses when the engine (1) is running in the diagnosis region. When SOx discharge control is performed, primary diagnosis and secondary diagnosis are prohibited, and if it is determined in the primary diagnosis that the front catalyst (8) has deteriorated, SOx discharge control is prohibited to provide an opportunity for secondary diagnosis. In this way, the deterioration diagnosis of the front catalyst (8) can be precisely performed even if there are few opportunities for diagnosis due to lean air-fuel ratio running or SOx discharge control.

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First Claim