Set Condition:

The conditions that cause this diagnostic to fail is when the upstream O2 sensor becomes 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 PCM 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 mass flow exceed the fail thresholds, the test fails and the diagnostic stops running for that trip. If the test fails on consecutive trips, a DTC is set.