The heated oxygen sensors (HO2S) are used for fuel control. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the engine is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module 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 control module, Closed Loop is entered. The control module 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.

The heating elements inside each HO2S heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Engine Control Module Connector End Views

Electrical Information Reference

Scan Tool Reference

Engine Control Module Scan Tool Information

Engine idling, observe the scan tool heated oxygen sensor voltage parameters. The reading should fluctuate above and below the range of 350-550 mV.

1. Ignition OFF, disconnect the harness connector at the applicable HO2S.
2. Test for less than 5 ohms between the HO2S heater ground circuit terminal C and ground.

⇒	If greater than the specified range, repair the ground circuit for an open/high resistance.

3. Test for less than 5 ohms between the low reference circuit terminal A and ground.
4. If greater than the specified range, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
5. Ignition ON, test for B+ between the HO2S heater ignition voltage circuit terminal D and ground.

⇒	If less than the specified range, repair the ignition voltage circuit for a short to ground or an open/high resistance.

6. Test for 400-500 mV between the signal circuit terminal B and ground.

⇒	If less than the specified range, test the signal circuit for an open or a short to ground. If the circuit tests normal, replace the ECM.

⇒	If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

7. If all circuits test normal, test or replace the applicable HO2S.

1. Ignition OFF, disconnect the harness connector at the applicable HO2S.
2. Test for 4-10 ohms between the ignition 1 voltage circuit terminal D and the ground circuit terminal C.

⇒	If not within the specified range, replace the HO2S sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.