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 provide an overview of each diagnostic category.

### DTC Descriptors

DTC P0131: HO2S Circuit Low Voltage Bank 1 Sensor 1
DTC P0151: HO2S Circuit Low Voltage Bank 2 Sensor 1

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
HO2S Bank 1 Sensor 1 Signal | P0131 | P0130, P0134 | P0132 | P0130, P0133
HO2S Bank 1 Sensor 2 Signal | P0137 | P0140 | P0138 | P0139
HO2S Bank 2 Sensor 1 Signal | P0151 | P0150, P0154 | P0152 | P0150, P0153
HO2S Bank 2 Sensor 2 Signal | P0157 | P0160 | P0158 | P0159
Low Reference | -- | P0130, P0150 | P0134, P0140, P0154, P0160 | --

### 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 more 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. If the ECM detects that the HO2S signal voltage is low, this DTC sets.

### Conditions for Running the DTC

• | The engine is operating.

• | The ignition 1 voltage is between 10.7-18 volts.

• | DTCs P0131 and P0151 run continuously once the above conditions are met for more than 2 seconds.

### Conditions for Setting the DTC

The internal ECM HO2S voltage is less than a threshold for more than 4 seconds.

### Action Taken when the DTC Sets

DTCs P0131 and P0151 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0131 and P0151 are Type B DTCs.

### Diagnostic Aids

• | The front wide band sensors do not toggle or switch like a switching HO2S. The front HO2S signals will be relatively stable for an idling engine.

• | A loose HO2S can cause this DTC to set.

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

#### HO2S Voltages

HO2S Circuit | Voltage
--- | ---

• | Ignition ON, Engine OFF

• | HO2S Disconnected

Heater Control | 4.6-5.0 V
Heater Supply Voltage | B+
Reference Voltage | 2.6-3.1 V
Low Reference | 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

• | Engine Control Module Connector End Views

• | Engine Controls 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

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Engine running at normal operating temperature, verify the scan tool Loop Status Bn. 1 Sen. 1 and Loop Status Bn. 2 Sen. 1 parameters display Closed Loop.
2. If the vehicle passes the Circuit/System Verification test, then operate the vehicle within the conditions for running the DTC. 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 sensor.
2. Ignition ON, test for 350-550 mV between the reference voltage circuit and the low reference circuit.

⇒ | If less than the specified range, test the reference voltage circuit and the low reference circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the ECM.

⇒ | If greater than the specified range, test the reference voltage circuit and the low reference circuit for a short to voltage. If the circuits test normal, replace the ECM.

3. Connect a test lamp between the input pump current circuit and B+.

⇒ | If the test lamp illuminates, test the input pump current circuit and the pump current circuit for a short to ground. If the circuits test normal, replace the ECM.

4. Verify that the following conditions do not exist:

• | Lean fuel injectors--Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2

• | 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.

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

### 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 1 Sensor 2

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