– Heated Oxygen Sensor harness is open or shorted
– Heated Oxygen Sensor circuit poor electrical connection
– Exhaust leaks
– Contaminated Heated Oxygen Sensor
The conditions that caused this diagnostic to fail is when the upstream O2 Sensor biased from an exhaust leak, O2 sensor contamination, or some other extreme operating condition. The downstream O2 sensor is considered to be protected from extreme environments by the catalyst. The Powertrain Control Module monitors the downstream O2 sensor feedback control, called downstream fuel trim, to detect any shift in the upstream O2 sensor target voltage from nominal target voltage. The value of the downstream fuel trim is compared with the lean thresholds. Every time the value exceeds the calibrated threshold, a fail timer is incremented and mass flow through the exhaust is accumulated. If the fail timer and accumulated mas flow exceed the fail thresholds, the test fails and the diagnostic stops running for that trip. If the test fail on consecutive trips, the code will set.