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: HO2S Reference Ground Circuit Bank 1 Sensor 1
DTC P2254: HO2S Reference Ground Circuit Bank 2 Sensor 1

### Circuit Description

The wide band heated oxygen sensor (HO2S) measures the amount of oxygen in the exhaust system and provides more information than the switching style HO2S. The wide band sensor consists of an oxygen sensing cell, an oxygen pumping cell, and a heater. The exhaust gas sample passes through a diffusion gap between the sensing cell and the pumping cell. The engine control module (ECM) supplies a voltage to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. An electronic circuit within the ECM controls the pump current through the oxygen pumping cell in order to maintain a constant voltage in the oxygen sensing cell. The ECM monitors the voltage variation in the sensing cell and attempts to keep the voltage constant by increasing or decreasing the amount of current flow, or oxygen ion flow, to the pumping cell. By measuring the amount of current required to maintain the voltage in the sensing cell, the ECM can determine the concentration of oxygen in the exhaust. The HO2S voltage is displayed as a lambda value. A lambda value of 1 is equal to a stoichiometric air fuel ratio of 14.7:1. Under normal operating conditions, the lambda value will remain around 1. When the fuel system is lean, the oxygen level will be high and the lambda signal will be high or greater than 1. When the fuel system is rich, the oxygen level will be low, and the lambda signal will be low or less than 1. The ECM uses this information to maintain the correct air/fuel ratio.

### Conditions for Running the DTC

• | Before the ECM can report DTC P2251 or P2254 failed, DTC P0030, P0031, P0032, P0050, P0051, P0052, P0135 and P0155 must run and pass.

• | The ignition 1 voltage is between 11-18 volts for greater than 30 seconds.

• | The HO2S heater control is enabled.

• | The ECM has not commanded fuel cut-off for greater than 2 seconds.

• | The HO2S heater is at operating temperature for greater than 20 seconds.

• | The internal HO2S sensing element resistance is greater than 570 ohms.

• | The calculated exhaust temperature is less than 900°C (1,652°F).

• | The ECM detects the internal HO2S signal voltage is between 1.47-1.53 volts.

• | DTC P2251 and P2254 run continuously once the above conditions are met for greater than 10 seconds.

### Conditions for Setting the DTC

• | The ECM detects the internal HO2S voltage signal change is greater than 0.12 mV as the heater control switches.

• | The above condition occurs 20 times within 10 seconds.

### Action Taken when the DTC Sets

DTC P2251 and P2254 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTC P2251 and P2254 are Type B DTCs.

### Diagnostic Aids

• | The front wide band sensor does not toggle or switch like a switching HO2S. The front HO2S signal will be relatively stable for an idling engine.

• | The following table illustrates the typical voltages for the HO2S circuits:

HO2S Circuit | Voltage
--- | ---

• | Ignition ON, Engine OFF

• | HO2S Disconnected

Heater Control | 4.6-5.0 V
Heater Supply Voltage | B+
High Signal | 2.6-3.1 V
Low Signal | 2.2-2.7 V
Pump Current | 2.5-3.0 V
Input Pump Current | 2.5-3.0 V

### 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. Ignition ON, observe the scan tool DTC information. Verify that DTC P0131, P0132, P0151, P0152, P2243, or P2247 is not set.

⇒ | If any of the above DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

2. Engine idling at operating temperature. Observe the appropriate scan tool Loop Status Sen 1 parameter. The parameter should display Closed Loop.
3. 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 are captured in the Freeze Frame/Failure Records Data List.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the appropriate HO2S 1.
2. Ignition ON, test for 350-550 mV between the high signal circuit terminal 3 and the low signal circuit terminal 1.

⇒ | If less than the specified range, test the low signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

3. If all circuits test normal, replace the appropriate HO2S 1.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor Replacement - Bank 1 Sensor 1

• | Heated Oxygen Sensor Replacement - Bank 2 Sensor 1

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