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 provides 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

#### HO2S 1

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Signal | P0131, P0171 | P0132, P0134, P0137, P2A00 | P0132, P0134 | P0133, P1133, P2A00
Low Reference | -- | P0134, P0137 | -- | P0133, P1133, P2A00

#### HO2S 2

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Signal | P0137 | P0138, P0140 | P0138, P0140 | --
Low Reference | -- | P0138, P0140 | -- | --

### 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: 50-1,050 mV
HO2S 1 and 2 Signal | 0 mV | 1,275 mV | 1,275 mV
Low Reference | -- | 1,275 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. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,275 mV. 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 toward 1,000 mV indicates a rich fuel mixture. An HO2S voltage that decreases 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

• | DTCs P0030, P0036, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0201, P0202, P0203, P0204, P0443, P0496, P0502, and P2135 are not set.

• | The engine is operating.

• | The ignition voltage is between 10-18 volts.

• | The fuel system is in Closed Loop.

• | The vehicle is not decelerating.

• | The DTCs run continuously when the above conditions are met for 5 seconds.

### Conditions for Setting the DTC

#### P0131 or P0137

The engine control module (ECM) detects that the HO2S voltage is less than 50 mV for greater than 38 seconds for DTC P0131 or 43 seconds for DTC P0137.

#### P0132 or P0138

The ECM detects that the HO2S voltage is greater than 1,050 mV for greater than 10 seconds.

#### P0134 or P0140

The ECM detects that the HO2S voltage is greater than 1,700 mV for 20 seconds.

### Action Taken when the DTC Sets

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

### Diagnostic Aids

#### HO2S Terminal Identification

Lock Tab
1 | 2
3 | 4

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component 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

Control Module References for scan tool information

### Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. DTC P0030, P0036, P0053, P0054, P0135, or P0141 should not be set.

⇒ | If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

Note: A front HO2S circuit condition can cause a rear HO2S DTC to set.

2. Engine running, observe the appropriate scan tool HO2S voltage parameter. The reading should be within the range of 50-1,050 mV.
3. 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.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the appropriate HO2S.
2. Ignition OFF for 1 minute, test for less than 5 Ω between the low reference circuit terminal 3 and ground.

⇒ | 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, test for 1.5-2.1 V between the signal circuit terminal 1 and ground.

⇒ | If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. 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 3 A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 3. Engine running, verify the scan tool HO2S parameter is less than 60 mV.

⇒ | If greater than the specified value, 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 appropriate HO2S.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Caution: When replacing the HO2S perform the following:

• | A code clear with a scan tool, regardless of whether or not a DTC is set

• | HO2S heater resistance learn reset with a scan tool, where available

Perform the above in order to reset the HO2S resistance learned value and avoid possible HO2S failure.

• | Heated Oxygen Sensor 1 Replacement

• | Heated Oxygen Sensor 2 Replacement

• | Control Module References for ECM replacement, setup, and programming