CATALYST DETERIORATION DIAGNOSIS DEVICE FOR INTERNAL COMBUSTION ENGINE

In an internal combustion engine including an EGR device with an EGR catalyst in some of exhaust systems, an oxygen occlusion capacity of a catalyst disposed in an exhaust collecting pipe of it is obtained with high precision, and based on the result, deterioration of the catalyst can be accurately diagnosed. A catalyst deterioration diagnostic device for an internal combustion engine of the present invention implements active air-fuel ratio control which forcefully changes an air-fuel ratio of an exhaust gas which flows into a main catalyst between a lean side and a rich side with stoichiometry as a center. The catalyst deterioration diagnostic device measures an oxygen occlusion capacity of the main catalyst by using signals outputted from an air-fuel ratio sensor upstream from the main catalyst and an oxygen sensor downstream from the main catalyst at a time of implementation of the active air-fuel ratio control, and diagnoses deterioration of the main catalyst from the measurement result of the oxygen occlusion capacity. In the active air-fuel ratio control according to the catalyst deterioration diagnostic device of the present invention, when the EGR device with the EGR catalyst is stopped, a target air-fuel ratio of a cylinder group with the EGR device not provided in an exhaust system is changed with a large amplitude between the lean side and the rich side with stoichiometry as a center, and a target air-fuel ratio of a cylinder group with the EGR device provided in an exhaust system is kept stoichiometric.