Apparatus for diagnosing exhaust gas purifying device

Abstract

In an apparatus for diagnosing an exhaust gas purifying device, a catalyst deterioration level detectordetects the deterioration level of a catalyst from a relationship between a catalyst-upstream air-fuel-ratio detectorand a catalyst-downstream air-fuel-ratio detector. A catalyst-deterioration-level correction sectioncorrects the catalyst deterioration level detected by the catalyst deterioration level detectorbased on a falling response ratio representing the ratio between a falling response index of the catalyst-upstream air-fuel-ratio detectorand a falling response index of the catalyst-downstream air-fuel-ratio detectoror based on a rising response ratio representing the ratio between a rising response index of the catalyst-upstream air-fuel-ratio detectorand a rising response index of the catalyst-downstream air-fuel-ratio detector. A catalyst deterioration judgment sectiondetermines that the catalyst is deteriorated from information provided by the catalyst-deterioration-level correction section. This allows any deteriorated catalyst to be precisely detected even if an air-fuel ratio sensor downstream of the catalyst is degraded in terms of rising response.