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 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.

The HO2S utilizes the following circuits:

Closed Loop Test

Power Enrichment Test

Closed Loop Test

Power Enrichment Test

DTCs P0137 and P0138 are Type B DTCs.

DTCs P0137 and P0138 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, observe the scan tool HO2S voltage parameter. The reading should fluctuate above and below the range of 350-550 mV.
2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Ignition OFF, disconnect the harness connector at the HO2S sensor.
2. Ignition ON, test for less than 5 ohms of resistance between the low reference circuit terminal listed below and ground.

•	HO2S sensor 1 low reference circuit terminal 1

•	Euro IV HO2S sensor 1 low reference circuit terminal 3

⇒	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.

3. Ignition ON, verify the scan tool HO2S parameter is between 350-500 mV.

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

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

4. Install a 3A fused jumper wire between the signal circuit and the low reference circuit terminals listed below. Verify the scan tool HO2S sensor parameter is less than 60 mV.

•	HO2S sensor 1 low reference circuit terminal 1

•	Euro IV HO2S sensor 1 low reference circuit terminal 3

•	HO2S sensor 1 signal circuit terminal 2

•	Euro IV HO2S sensor 1 signal circuit terminal 1

⇒	If greater than the specified range, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

5. Verify the following conditions do not exist:

•	Lean fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .

•	Low fuel system pressure--Refer to Fuel System Diagnosis .

•	Fuel that is contaminated--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

•	Exhaust leaks near the HO2S

•	Engine vacuum leaks

⇒	If you find any of the above conditions, repair as necessary.

6. If all circuits test normal, test or replace the HO2S sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.