Device for judging deterioration of catalyst of engine

摘要

A device for judging the deterioration of a three-way catalyst comprising an upstream O2 sensor and a downstream O2 sensor which are arranged in the exhaust passage upstream and downstream of the three-way catalyst respectively. When the ratio of length of the output signal response curve of the downstream O2 sensor and the length of the output signal response curve of the upstream O2 sensor is over a threshold level, it is judged that the three-way catalyst has deteriorated. The threshold value is reduced as the mean value of the air-fuel ratio becomes off from the stoichiometric air-fuel ratio.