Patent ID: 7513105

Claim:
An exhaust gas purifying system, comprising: a catalyst arranged on an exhaust pipe of an internal combustion engine; a sensor detecting a state of the exhaust gas passed through said catalyst; and an abnormality determining apparatus determining abnormality of the system by monitoring an output of said sensor in an operation under forced air-fuel ratio control executed when said internal combustion engine satisfies a prescribed condition, in which air-fuel ratio of intake air supplied to said internal combustion engine is forced to increase/decrease; wherein when said operation under forced air-fuel control is interrupted before completion of the determination of abnormality of said system, said abnormality determining apparatus once stores a parameter accumulated during said operation under forced air-fuel ratio control by the time of interruption, and when said internal combustion engine comes to satisfy said prescribed condition again, resumes said operation under forced air-fuel ratio control, and completes determination of said system abnormality using said stored parameter and parameter accumulated after resumption of the forced air-fuel ratio control.