Patent ID: 8534046

Claim:
An apparatus for diagnosing deterioration of a catalyst provided in an exhaust passage of an internal combustion engine, the apparatus comprising: an upstream air-fuel ratio sensor provided upstream of the catalyst; a downstream air-fuel ratio sensor provided downstream of the catalyst; and an electronic control unit that includes program logic that: performs stoichiometric feedback control on an air-fuel ratio of exhaust gas that flows into the catalyst by controlling a fuel injector, based on at least an output from the upstream air-fuel ratio sensor; measures an oxygen storage capacity of the catalyst; and in case a measured value of the oxygen storage capacity of the catalyst is equal to or less than a predetermined deterioration determination value; calculates a ratio of a time period of lean output of the downstream air-fuel ratio sensor, which is leaner than a predetermined value, to a predetermined time period; corrects the measured value of the oxygen storage capacity of the catalyst, based on the ratio; and determines whether the catalyst is deteriorated by comparing the corrected measured value of the oxygen storage capacity with the predetermined deterioration determination value.