CATALYST DETERIORATION DIAGNOSIS METHOD AND CATALYST DETERIORATION DIAGNOSIS SYSTEM

A catalyst deterioration diagnosis method is a method for a system. The system includes a stepped transmission or a continuously variable transmission connected to an internal combustion engine, a catalyst into which an exhaust gas from the internal combustion engine is introduced, and a gas sensor having sensitivity to ammonia that outputs a detection value corresponding to a component of an exhaust gas that has passed through the catalyst. The catalyst deterioration diagnosis method includes the following steps. Monitoring of temporary increase of a detection value of the gas sensor is started, and thereby a temporarily increased amount of the detection value of the gas sensor is acquired. The monitoring is started when upshifting of the stepped transmission or pseudo-upshifting of the continuously variable transmission is performed. Whether or not the temporarily increased amount is larger than a threshold amount is determined.