Diagnostic Instructions

DTC Descriptors

DTC P0131: HO2S Circuit Low Voltage Sensor 1

DTC P0132: HO2S Circuit High Voltage Sensor 1

DTC P0134: HO2S Circuit Insufficient Activity Sensor 1

DTC P0137: HO2S Circuit Low Voltage Sensor 2

DTC P0138: HO2S Circuit High Voltage Sensor 2

DTC P0140: HO2S Circuit Insufficient Activity Sensor 2

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
HO2S 1 Signal	P0131, P0137	P0134, P0140	P0132, P0138	P0133, P0139
Low Reference	--	P0134, P0140	P0132, P0138	P0133, P0139

Typical Scan Tool Data

HO2S 1 and 2

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: Engine operating in Closed Loop. Parameter Normal Range: Fluctuates above and below 350-500 mV.
HO2S 1 and 2 Signal	0 mV	400-415 mV	1000 mV
Low Reference	--	400-415 mV	1000 mV

Circuit Description

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:

Conditions for Running the DTC

Conditions for Setting the DTC

P0131 or P0137

The engine control module (ECM) detects that the HO2S voltage is less than 50 mV.

P0132 or P0138

The ECM detects that the HO2S voltage is greater than 950 mV.

P0134 or P0140

The ECM detects that the HO2S voltage is between 350-550 mV.

Action Taken when the DTC Sets

DTCs P0131, P0132, P0134, P0137, P0138, and P0140 are Type A DTCs.

DTCs P0131, P0132, and P0134

The ECM commands open loop fuel control.

Conditions for Clearing the MIL/DTC

DTCs P0131, P0132, P0134, P0137, P0138, and P0140 are Type A DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Circuit/System Verification

Important: In extreme cold weather conditions ice may form in the vacuum source to the MAP sensor and may set a DTC. Inspect the MAP sensor and the vacuum source for ice, moisture, or other restrictions.

1. Ignition ON, observe the scan tool MAP sensor pressure parameter. Start the engine, verify the scan tool MAP sensor pressure parameter changes.

⇒	If the scan tool MAP sensor pressure parameter does not change, refer to DTC P0107 or P0108 for further diagnosis.

2. Engine idling, observe the applicable heated oxygen sensor voltage parameter. The reading should fluctuate above and below the range of 350-550 mV.
3. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector of the applicable HO2S.
2. Engine idling, verify that the applicable 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/connections test normal, replace the ECM.

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

3. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit and ground.

⇒	If greater than the specified value, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit/connections test normal, replace the ECM.

4. Install a 3-amp fused jumper wire between the signal circuit and the low reference circuit. Engine idling, verify the HO2S parameter is less than 60 mV.

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

5. Verify that 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/connections test normal, replace the applicable HO2S.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.