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 P0138: HO2S Circuit High Voltage Bank 1 Sensor 2
DTC P0158: HO2S Circuit High Voltage Bank 2 Sensor 2

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Bank 1 Sensor 1 Signal | P0131 | -- | P0132 | P0133, P0134
HO2S Bank 1 Sensor 2 Signal | P0137 | -- | P0138 | P0139, P0140
HO2S Bank 2 Sensor 1 Signal | P0151 | -- | P0152 | P0153, P0154
HO2S Bank 2 Sensor 2 Signal | P0157 | -- | P0158 | P0159, P0160
Low Reference | -- | -- | P0132, P0138, P0152, P0158 | --

### 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 | 400-430 mV | 5000 mV
Low Reference | -- | 400-430 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.

The HO2S utilizes the following circuits:

• | A signal circuit

• | A low reference circuit

• | An ignition 1 voltage circuit

• | A heater control circuit

### Conditions for Running the DTCs

• | DTCs P0131, P0132, P0133, P0134, P0137, P0139, P0140, P0151, P0152, P0153, P0154, P0157, P0159, and P0160 are not set.

• | The ignition is ON.

• | The ignition 1 voltage is between 10-16 volts.

• | The DTCs run continuously once the above conditions are met.

### Conditions for Setting the DTCs

The engine control module (ECM) detects that the HO2S voltage is greater than 1 volt greater than 2 seconds.

### Action Taken when the DTC Sets

DTCs P0138 and P0158 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0138 and P0158 are Type B DTCs.

### 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. Engine running, observe the scan tool HO2S voltage parameter. The reading should fluctuate above and below 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.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the appropriate HO2S sensor.
2. Ignition ON, test for less than 0.1 volt 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. 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 for a short to voltage. If the circuit tests normal, replace the ECM.

4. Install a 3A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 3. Verify the scan tool HO2S sensor parameter is less than 60 mV.

⇒ | 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:

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

• | High fuel system pressure--Refer to Fuel System Diagnosis.

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

• | Restricted exhaust

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

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

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor Replacement - Bank 1 Sensor 2

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