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 P2251: O2 Sensor Ground Circuit Malfunction (Bank 1 Sensor 1)
DTC P2254: O2 Sensor Ground Circuit Malfunction (Bank 2 Sensor 1)

### 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

### 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 engine control module (ECM) operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air/fuel ratio. The ECM 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 ECM, Closed Loop is entered. The v uses the HO2S voltage to determine the air/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 ECM to calculate the air/fuel ratio sooner.

### Conditions for Running the DTCs

#### DTC P2251,or P2254

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

• | The engine is operating.

### Conditions for Setting the DTCs

#### DTC P2251,or P2254

The ECM detects that the HO2S low reference circuit is low.

### Action Taken when the DTC Sets

DTC P2251,or P2254 are type B DTCs.

### Conditions for Clearing the MIL/DTC

DTC P2251,or P2254 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

### Circuit/System Verification

• | Allow the engine to reach operating temperature. Engine running, using a scan tool observe the affected HO2S parameter.

- | The HO2S 1 value should vary from below 200 mV to above 800 mV and respond to fuelling changes.

- | The HO2S 2 value should change more than 200 mV when the throttle is quickly cycled 3 times from Closed to wide open and back to Closed after running the engine at 1,500 RPM for 30 seconds.

• | If any HO2S heater DTCs are set, diagnose those DTCs first.

• | The affected HO2S may be damaged due to contamination. Prior to replacing the affected HO2S, inspect for the following sources of contamination:

⇒ | Silicon contamination.

⇒ | Engine oil contamination.

⇒ | Engine coolant contamination.

### Circuit/System Testing

1. Disconnect the affected HO2S.
2. Install a 3-A fusible link wire between the low reference signal circuit and the a low reference return circuit.

⇒ | If voltage does not read 1 V. test the circuit for short or open/high resistance. If circuit test normal replace the ECM.

3. If the ECM and all circuits test normal, replace the affected HO2S.

### Repair Instructions

Perform the Diagnostic Repair Verification.

• | Heated Oxygen Sensor Replacement - Bank 1 Sensor 1

• | Heated Oxygen Sensor Replacement - Bank 2 Sensor 2

• | Engine Control Module Replacement for engine control module replacement, setup and programming.