For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provide an overview of each diagnostic category.

### 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 Bank 1 Sensor 1 Signal | P0131 | P0134 | P0132 | P0133
HO2S Bank 1 Sensor 2 Signal | P0137 | P0140 | P0138 | --
Low Reference | -- | P0134 | P0134, P0140 | --

### Typical Scan Tool Data

#### HO2S 1 or 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
Sensor Signal | 0-60 mV | 400-415 mV | 5000 mV
Low Reference | -- | 400-415 mV | 5000 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.

### Conditions for Running the DTC

#### P0131 and P0132

• | The HO2S 2 sensor voltage is greater than 0.5 volt.

• | DTCs P0131 and P0132 run once an ignition cycle when the above condition is met.

#### P0134

• | The fuel cut-off time is greater than 3 seconds.

• | The exhaust temperature is greater than 600°C (1112°F).

• | The HO2S 2 voltage is greater than 0.2 volt.

• | DTC P0134 runs continuously once the above conditions are met.

#### P0137 and P0138

• | The engine is running in closed loop.

• | DTC P0137 and P0138 run once an ignition cycle when the above condition is met.

#### P0140

• | The exhaust temperature is greater than 600°C (1112°F).

• | DTC P0140 runs once an ignition cycle when the above condition is met.

### Conditions for Setting the DTC

#### P0131

The HO2S 1 sensor voltage is less than 1 volt

#### P0132

The HO2S 1 sensor voltage is greater than 2 volts

#### P0134

The ECM detects the HO2S 1 internal resistance is greater than 20,000 ohms when the sensor voltage is between 0.4-0.6 volt.

#### P0137

The HO2S 2 sensor voltage is less than 1 volt

#### P0138

The HO2S 2 sensor voltage is greater than 2 volts

#### DTC P0140

The ECM detects the HO2S 2 internal resistance is greater than 20,000 ohms when the sensor voltage is between 0.4-0.6 volt.

### Action Taken when the DTC Sets

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

### Conditions for Clearing the MIL/DTC

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

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Engine idling, observe the scan tool HO2S voltage parameter. The voltage should fluctuate above and below the range of 350-550 mV.
2. 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, and the MAIN fuse removed, disconnect the harness connector at the appropriate HO2S sensor.
2. Ignition ON, verify the scan tool HO2S voltage 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.

3. Install the MAIN fuse. Ignition OFF, test for 5 ohms or less between the low reference circuit and ground.

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

4. Ignition ON, install a 3A fused jumper wire between the signal circuit and the low reference circuit. Verify the scan tool HO2S voltage parameter is less than 60 mV.

⇒ | If greater than the specified range, test the signal circuit of the HO2S sensor 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 or Fuel Injector Balance Test with Tech 2

• | 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, replace the appropriate HO2S sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor 1 Replacement

• | Heated Oxygen Sensor 2 Replacement

• | Control Module References for engine control module replacement, setup, and programming