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 P0137: HO2S Circuit Low Voltage Sensor 2
DTC P0138: HO2S Circuit High Voltage Sensor 2

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Sensor 1 Signal | P0131 | P0131 | P0132 | P0133
HO2S Sensor 2 Signal | P0137 | P0137 | P0138 | P0140
Low Reference | -- | P0131, P0137 | P0132, P0138 | --

### 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*
* Internal ECM damage may occur if the circuit is shorted to B+.

### Circuit/System 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 DTC

#### Closed Loop Test

• | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0336, P0337, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, and P1404 are not set.

• | The engine coolant temperature is more than 60°C (140°F).

• | The ignition 1 voltage is more than 10 volts.

• | The calculated airflow is more than 8 g/s.

• | The fuel system is in closed loop.

• | The DTCs run continuously once the above conditions are met for 3 seconds.

#### Power Enrichment Test

• | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0336, P0337, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, and P1404 are not set.

• | The engine coolant temperature is more than 60°C (140°F).

• | The ignition 1 voltage is more than 10 volts.

• | The air fuel ratio is less than or equal to 13.5:1.

• | The DTCs run continuously once the above conditions are met for 3 seconds.

• | The engine coolant temperature is more than 60°C (140°F).

#### Decel Fuel Cut-Off (DFCO) Test

• | DTCs P0106, P0107, P0108, P0117, P0118, P0122, P0123, P0141, P0171, P0172, P0201-P0204, P0300, P0336, P0337, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, and P1404 are not set.

• | The engine coolant temperature is more than 60°C (140°F).

• | The ignition 1 voltage is more than 10 volts.

• | DTC P0138 runs continuously once the above conditions are met for 3 seconds.

### Conditions for Setting the DTC

#### Closed Loop Test

• | The engine control module (ECM) detects that the HO2S 2 voltage is less than 52 mV for more than 4 minutes.

• | The ECM detects that the HO2S 2 voltage is more than 952 mV for more than 4 minutes.

#### Power Enrichment Test

• | The ECM detects that the HO2S 2 voltage is less than 35 mV during power enrichment for more than 11 seconds.

• | The ECM detects that the HO2S 2 voltage is more than 550 mV during deceleration for more than 11 seconds.

#### Decel Fuel Cut-Off (DFCO) Test

• | The ECM detects that the HO2S 2 voltage is more than 550 mV during deceleration.

• | The condition exists for more than 11 seconds.

### Action Taken when the DTC Sets

DTCs P0137 and P0138 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0137 and P0138 are Type B DTCs.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### 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 the range of 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 HO2S sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal listed below and ground.

• | HO2S sensor 1 low reference circuit terminal 1

• | Euro IV HO2S sensor 1 low reference circuit terminal 3

⇒ | 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, 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 sensor for a short to voltage. If the circuit tests normal, replace the ECM.

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

• | HO2S sensor 1 low reference circuit terminal 1

• | Euro IV HO2S sensor 1 low reference circuit terminal 3

• | HO2S sensor 1 signal circuit terminal 2

• | Euro IV HO2S sensor 1 signal circuit terminal 1

⇒ | 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:

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

• | 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, test or replace the HO2S sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor 2 Replacement

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