Patent ID: 7430853

Claim:
An apparatus for evaluating the deterioration condition of a catalyst provided in an exhaust system of an internal combustion engine, the apparatus comprising: a catalyst downstream oxygen concentration detector that detects an oxygen concentration of exhaust gas downstream of the catalyst; and a controller that determines that an air/fuel ratio of exhaust gas downstream of the catalyst is in fuel rich condition when a detected value of the catalyst downstream oxygen concentration detector is more than a first value, determines that the air/fuel ratio of exhaust gas downstream of the catalyst is in fuel lean condition when a detected value of the catalyst downstream oxygen concentration detector is less than a second value, and controls the internal combustion engine to change an air/fuel ratio of exhaust gas upstream of the catalyst when an integrated value of an intake air volume of the internal combustion engine becomes a predetermined value, the integrated value being integrated until a second timing after the integrated value is reset at a first timing, the second timing being determined based on a determination result of the air/fuel ratio of exhaust gas downstream of the catalyst, wherein the first timing is when the determination result of the air/fuel ratio is changed, and the second timing is when the determination result of the air/fuel ratio is subsequently changed, the second timing occurring when the integrated value of the intake air volume of the internal combustion engine becomes the predetermined value.