Diagnostic system for diagnosing deterioration of heated type oxygen

Abstract

In a heated type oxygen sensor having a sensing element responding to changes in the exhaust oxygen content and a heater inside, a diagnostic system for diagnosing deterioration of the heated type oxygen sensor comprises an electronic control unit being responsive to an engine temperature for discriminating whether the engine is in a cold-engine state or in a fully warmed-up state. The diagnostic system includes a first warm-up time setting circuit for setting a predetermined oxygen sensor's warm-up time at a first delay time which is preset to be suitable for the cold-engine state, during cold-engine operations, and a second warm-up time setting circuit for setting the predetermined oxygen sensor's warm-up time at a second delay time which is preprogrammed to be suitable for the fully warmed-up state, during fully warmed-up engine operations. The diagnostic system makes a diagnosis on deterioration of the oxygen sensor under a particular condition in which an elapsed time, measured from a time when the heater is turned on, reaches the predetermined oxygen sensor's warm-up time set depending on during cold-engine operation or during fully warmed up engine operation.