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 Mode - P0137 and P0138

Power Enrichment Mode - P0137

Decel Fuel Cut-Off (DFCO) Mode - P0138

The DTC runs continuously when the above conditions are met for 2 seconds.

P0137

The engine control module (ECM) detects that the HO2S 2 voltage is less than 52 mV during the closed loop mode or less than 350 mV during the power enrichment mode.

P0138

The ECM detects that the HO2S 2 voltage is greater than 952 mV during the closed loop mode or greater than 550 mV during the DFCO mode.

DTCs  P0137 and P0138 are Type B DTCs.

DTCs  P0137 and P0138 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

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 heated oxygen sensor (HO2S) 2 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 2.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit and ground.

•	HO2S 2 low reference circuit terminal 3

•	HO2S 2 (Mexico, Japan) low reference circuit terminal 3

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

3. Ignition ON, verify the applicable scan tool HO2S 2 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 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. Verify the scan tool HO2S 2 parameter is less than 60 mV.

•	HO2S 2 signal circuit terminal 1

•	HO2S 2 (Mexico, Japan) signal circuit terminal 2

⇒	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 none of the following conditions exist:

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

•	Water intrusion in the HO2S harness connector

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

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

•	Fuel saturation of the evaporative emission (EVAP) canister

•	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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.