ENGINE CATALYST DIAGNOSIS APPARATUS

An engine catalyst diagnosis apparatus includes an exhaust passage for discharging exhaust gas from a combustion chamber of an engine, a catalyst device provided on the exhaust passage for purifying the exhaust gas, a linear A/F sensor that is provided on the exhaust passage downstream of the catalyst device and is configured to detect the air-fuel ratio of the exhaust gas, and a processing device configured to diagnose the catalyst device on the basis of the air-fuel ratio detected by the linear A/F sensor, and at start of the engine after soaking, the processing device acquires the temperature of the catalyst device, calculates a fluctuation range of the air-fuel ratio detected by the linear A/F sensor within a predetermined time, and diagnoses the catalyst device on the basis of a relationship between the fluctuation range and the catalyst temperature.