Patent ID: 7293404

Claim:
A control device for internal combustion engine comprising: a ternary catalyst arranged in an exhaust system of an internal combustion engine; an upstream air-fuel ratio detecting unit that is arranged in the exhaust system upstream from the ternary catalyst and detects an upstream air-fuel ratio of exhaust gas of the internal combustion engine; an upstream air-fuel ratio feedback control unit that performs feedback control of the quantity of fuel injection to the internal combustion engine so that the upstream air-fuel ratio detected by the upstream air-fuel ratio detecting unit coincides with an upstream target air-fuel ratio; a downstream air-fuel ratio detecting unit that is arranged in the exhaust system downstream from the ternary catalyst and detects a downstream air-fuel ratio of the exhaust gas; a downstream air-fuel ratio feedback control unit that performs feedback control of the upstream target air-fuel ratio so that the downstream air-fuel ratio detected by the downstream air-fuel ratio detecting unit coincides with a downstream target air-fuel ratio; a target air-fuel ratio varying unit that varies the upstream target air-fuel ratio at the time of diagnosing degradation of the ternary catalyst; and a catalyst degradation judging unit that judges degradation of the catalyst from the behavior of the downstream air-fuel ratio at the time of diagnosing degradation of the ternary catalyst; wherein at the time of diagnosing degradation of the ternary catalyst, the downstream air-fuel ratio feedback control unit is stopped or its control constant is set at a smaller value.