[amidgeha]

Post Catalyst Fuel Trim System Too Lean

The Heated Oxygen Sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the engine is started, the engine control module (ECM) operates in an open loop mode, ignoring the HO2S signal voltage while calculating the air to fuel ratio. The ECM supplies the HO2S with a reference, or bias voltage of about 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1 000 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the ECM, closed loop is entered. The ECM uses the HO2S voltage to determine the air to fuel ratio. An HO2S voltage that increases above bias voltage toward 1 000 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.

Cause:
Faulty Heated Oxygen Sensor
Heated Oxygen Sensor harness is open or shorted
Heated Oxygen Sensor circuit poor electrical connection
Exhaust leaks
Contaminated Heated Oxygen Sensor

Symptoms:
Check engine light