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 P1137: HO2S Circuit Low Voltage During Power Enrichment Sensor 1
DTC P1166: HO2S Circuit Low Voltage During Power Enrichment Sensor 2

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Signal | P0131, P0137, P1137, P1166 | P0134, P0140 | P0132, P0138, P1138, P2297 | P0133
Low Reference | -- | P0134, P0140 | P0132, P0138, P1138, P2297 | P0133

### 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: Fluctuates above and below 350-500 mV
HO2S 1 and 2 Signal | 0 mV | 400-415 mV | 1000 mV
Low Reference | -- | 400-415 mV | 1000 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 DTC

• | The engine airflow is greater than 5 g/s.

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

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

• | The engine has been running for 60 seconds.

• | The engine is in power enrichment (PE) mode.

• | The DTCs run continuously when the above conditions are met for 2 seconds.

### Conditions for Setting the DTC

The engine control module (ECM) detects that the HO2S voltage is less than 350 mV and the air/fuel ratio is less than 13.5:1 during PE mode.

### Action Taken when the DTC Sets

• | DTC P1137 is type C DTC.

• | DTC P1166 is type B DTC.

### Conditions for Clearing the MIL/DTC

• | DTC P1137 is type C DTC.

• | DTC P1166 is type B DTC.

### 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 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 appropriate HO2S.
2. Engine running, verify the scan tool HO2S parameter is between 350-500 mV.

⇒ | If less than the specifed range, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.

3. Verify that none of the following conditions exist:

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

• | Water intrusion in the HO2S harness connector

• | 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

• | Contaminated HO2S--Silicon

• | Engine vacuum leaks

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

4. If all circuits test normal, test or replace the applicable HO2S.

### 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 ECM replacement, setup, and programming