Patent ID: 8011174

Claim:
An apparatus for diagnosing an exhaust gas purifying device used in an internal combustion engine which provides an air-fuel ratio control unit that detects the air-fuel ratio of an exhaust gas of the internal combustion engine and regulates a fuel injection amount so as to maintain the air-fuel ratio of the exhaust gas at a predetermined value, the apparatus comprising: a catalyst-upstream air-fuel-ratio detection means disposed upstream of a catalyst; a catalyst-downstream air-fuel-ratio detection means disposed downstream of the catalyst; and a catalyst-deterioration-level detection means for detecting a catalyst deterioration level from a relationship between the catalyst-upstream air-fuel-ratio detection means and the catalyst-downstream air-fuel-ratio detection means; the apparatus further comprising: a catalyst-deterioration-level correction means for correcting the catalyst deterioration level detected by the catalyst-deterioration-level detection means based on a falling response ratio representing the ratio between a falling response index of the catalyst-upstream air-fuel-ratio detection means and a falling response index of the catalyst-downstream air-fuel-ratio detection means or based on a rising response ratio representing the ratio between a rising response index of the catalyst-upstream air-fuel-ratio detection means and a rising response index of the catalyst-downstream air-fuel-ratio detection means; and a catalyst deterioration judgment means for determining that the catalyst is deteriorated based on information provided by the catalyst-deterioration-level correction means.