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 P0031: HO2S Heater Control Circuit Low Voltage Sensor 1
DTC P0032: HO2S Heater Control Circuit High Voltage Sensor 1
DTC P0037: HO2S Heater Control Circuit Low Voltage Sensor 2
DTC P0038: HO2S Heater Control Circuit High Voltage Sensor 2

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Ignition 1 Voltage | * | P0031, P0037 | -- | --
HO2S 1 Heater Control | P0031 | P0031, P0135 | P0032 | P0135
HO2S 2 Heater Control | P0037 | P0037, P0141 | P0038 | P0141
* Engine cranks but does not run and the fuse for the ignition circuit opens.

### Circuit Description

The heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. A heating element inside the HO2S minimizes the time required for the sensor to reach operating temperature. Voltage is provided to the heater by the ignition 1 voltage circuit through a fuse. Ground is provided to the heater on the low control circuit by a low side driver within the control module.

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 DTC

• | The engine is running.

• | The ignition 1 voltage is between 7.5-16 volts.

• | The DTC runs continuously when the above conditions are met.

### Conditions for Setting the DTC

The engine control module (ECM) detects an open, short to ground, or short to voltage on the control circuit of the H02S.

### Action Taken When the DTC Sets

• | DTC P0031, P0032, P0037, and P0038 are Type A DTCs.

• | The ECM commands the fuel control system to open loop.

### Conditions for Clearing the MIL/DTC

DTC P0031, P0032, P0037, and P0038 are Type A 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

Important: It may take up to 8 minutes for the DTC to set.

1. Engine at normal operating temperature, operate the engine above 1,200 RPM for 30 seconds and then allow the engine to idle. DTC P0031, P0032, P0037, or P0038 should not set.
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 heated oxygen sensor (HO2S).
2. Ignition ON, verify that a test lamp illuminates between the ignition 1 voltage circuit and ground.

• | HO2S ignition 1 voltage circuit terminal 2

• | HO2S (Mexico, Japan) ignition 1 voltage circuit terminal 1

⇒ | If the test lamp does not illuminate, test the ignition 1 voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition 1 voltage circuit fuse is open, test all components connected to the ignition 1 voltage circuit and replace as necessary.

3. Ignition OFF, connect a test lamp between the control circuit and 12 volts.

• | HO2S control circuit terminal 4

• | HO2S (Mexico, Japan) ignition 1 voltage circuit terminal 3

4. Ignition ON, command the HO2S Heater ON and OFF with a scan tool. The test lamp should turn ON and OFF.

⇒ | If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

⇒ | If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.

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

### Component Testing

1. Ignition OFF, disconnect the harness connector at the HO2S.
2. Test for 3-23 ohms of resistance between the ignition 1 voltage terminal and the control terminal.

⇒ | If not within the specified range, replace the 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