Patent ID: 7040085

Claim:
A deterioration detecting device for detecting deterioration of a downstream oxygen concentration sensor disposed downstream of a catalyst which is provided in an exhaust passage of an internal combustion engine to purify exhaust gases, said catalyst having a capability of storing oxygen, said deterioration detecting device comprising: execution condition determining means for determining an execution condition for deterioration determination of said downstream oxygen concentration sensor; rich air-fuel ratio setting means for setting an air-fuel ratio of an air-fuel mixture to be supplied to said engine to a predetermined rich air-fuel ratio in a rich region with respect to a stoichiometric ratio when the execution condition is satisfied; air-fuel ratio changing means for changing the air-fuel ratio to a predetermined lean air-fuel ratio in a lean region with respect to the stoichiometric ratio when a predetermined enrichment time period has elapsed from the time of enrichment of the air-fuel ratio by said rich air-fuel ratio setting means; dead time period measuring means for measuring a dead time period from the time the air-fuel ratio has been changed by said air-fuel ratio changing means to the time an output from said downstream oxygen concentration sensor reaches a first predetermined value; determination threshold setting means for setting a determination threshold according to the dead time period; change time period measuring means for measuring a change time period from the time the output from said downstream oxygen concentration sensor has reached the first predetermined value to the time the output from said downstream oxygen concentration sensor reaches a second predetermined value indicative of an oxygen concentration higher than an oxygen concentration corresponding to the first predetermined value; and deterioration determining means for determining that said downstream oxygen concentration sensor is deteriorated when the change time period is greater than the determination threshold.