Deterioration detecting device for oxygen concentration sensor

A measuring unit determines a change time period from the time the output of a downstream oxygen concentration sensor (15) has reached a first predetermined value to the time the output of the sensor reaches a second predetermined value. A determining unit detects whether the sensor has deteriorated when the change time period is greater than a determination threshold. Independent claims are also included for the following: (a) a sensor deterioration detecting method; and (b) a computer program.