Exhaust leaks before or near the Rear Heated Oxygen Sensor (HO2S)
Faulty Rear Heated Oxygen Sensor (HO2S)
Rear Heated Oxygen Sensor (HO2S) harness is open or shorted
Rear Heated Oxygen Sensor (HO2S) circuit poor electrical connection
Engine Light ON (or Service Engine Soon Warning Light)
During a Deceleration Fuel Shut-Off (DFSO) event, the Powertrain Control Module (PCM) monitors how quickly the rear heated oxygen sensor (HO2S) switches from rich to lean. The measured rate of the rich to lean switch is compared to a calibrated threshold value. The threshold value takes into account the level of oxygen in the catalyst, which has an impact on how quickly the rich to lean switch occurs. The test fails when the measured value is slower than the threshold value.