Diagnostic device and method of engine exhaust purifying system

The invention relates to a diagnostic device and method of an engine exhaust purifying system, which perform active control of an air-fuel ratio and can make diagnosis of air-fuel ratio sensors (81, 82), catalysts (70, 75), etc. in the engine exhaust purifying system with high accuracy and reliability while avoiding a worsening of exhaust emissions, an increase of revolution variations, etc. The diagnostic device comprises an exhaust air-fuel ratio control unit (100) for performing control to keep an air-fuel ratio of exhaust gas flowing into a main catalyst (75) near a stoichiometric ratio in accordance with outputs of downstream air-fuel ratio sensors (82), a rich/lean switching unit (102) for switching over an air-fuel ratio in each of the cylinder groups to be rich or lean, thereby making diagnosis of each downstream air-fuel ratio sensor (82), a response detecting unit (110) for detecting a response of the downstream air-fuel ratio sensor (82) when the rich/lean switching of the air-fuel ratio in each cylinder group is performed, and a downstream sensor deterioration determining unit (104) for determining a deterioration of the downstream air-fuel ratio sensor (82) based on the detected response.