GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 GTO 🢒 DTC 🢒 DTC P0131 or P0151

Circuit Description

The powertrain control module (PCM) supplies a voltage of about 450 mV between the heated oxygen sensor (HO2S) high and low signal circuits. The oxygen sensor (O2) varies the voltage from approximately 1,000 mV when the exhaust is rich, through approximately 10 mV when the exhaust is lean.

The PCM monitors and stores the HO2S voltage information. The PCM evaluates the HO2S voltage samples in order to determine the amount of time the HO2S voltage was out of range. The PCM compares the stored HO2S voltage samples taken within each sample period, and determines if the majority of the samples are out of the operating range.

The PCM monitors the HO2S voltage for being fixed below a predetermined voltage. If the PCM detects the voltage is below a predetermined voltage, a DTC sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs.

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

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0200, P0300, P0443, P0496, P1111, P1112, P1114, P1115, P1121, or P1122 are not set.
    • The ignition 1 voltage is between 10-18 volts.
    • The fuel level is more than 10 percent.
    • The loop status is CLOSED.
    • Intrusive tests are not in progress.
    • DTC P0131 and P0151 run continuously when the above conditions are met.

Lean Test Enable

    • The air fuel ratio is between 14.5:1 and 14.7:1.
    • The throttle position (TP) is between 3-70 percent.
    • The above conditions are met for at least 2 seconds.

Power Enrichment Test Enable

    • Power enrichment is active for at least 1 second.
    • The engine has been running for at least 30 seconds.

Conditions for Setting the DTC

Lean Test

The HO2S voltage is less than 200 mV for 165 seconds.

Power Enrichment Test

The HO2S voltage is less than 360 mV for 10 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
    • The control module commands Open Loop.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Important: 

   • Before you service the PCM, remove any debris from the PCM connector surfaces. Inspect the PCM connector gaskets when you diagnose or replace the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent contaminant intrusion into the PCM.
   • For any test that requires probing the PCM or component harness connectors, use the J 35616 Connector Test Adapter Kit . Using this kit prevents any damage to the harness connector terminals. Refer to Using Connector Test Adapters in Wiring Systems.

The PCM sets this DTC when the vehicle runs out of fuel. Inspect for the following conditions:

    • The heated oxygen sensor (HO2S) wires--The sensor pigtail may be in contact with the exhaust manifold or the exhaust system.
    • The oxygen supply--An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provide the supply of oxygen. Inspect the HO2S wires and connections for breaks or for contamination. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.
    • An intermittent ground--Inspect for an intermittent ground in the signal wire between the connector and the sensor.
    • Lean injectors--Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 or Fuel Injector Balance Test with Special Tool .
    • Fuel contamination--Water near the in-tank fuel pump inlet can be delivered to the injectors. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . The water causes a lean exhaust and can set a DTC.
    • The fuel pressure--If the fuel pressure is too low, the system will be lean. Refer to Fuel System Diagnosis .
    • Exhaust leaks--An exhaust leak near the HO2S can cause a lean condition. Refer to Exhaust Leakage in Engine Exhaust.
    • Vacuum or crankcase leaks can cause a lean condition.

If the above components are OK, the HO2S may be at fault.

For an intermittent condition, refer to Symptoms - Engine Controls .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. If the voltage is below the specified value, the condition is present.

    2.

  2. A disconnected sensor should indicate a bias voltage. If the voltage is low, inspect the signal circuit for a short to ground.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Start and run the engine until normal operating temperature is reached.
  2. With a scan tool, observe the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage less than the specified value?

200 mV

Go to Step 4

Go to Step 3

3
  1. Observe the Freeze Frame/Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the ignition.
  2. Disconnect the HO2S connector for the sensor that applies to this DTC.
  3. Jumper the HO2S low signal circuit terminal on the engine harness side to a known good ground.
  4. Turn ON the ignition, with the engine OFF.
  5. Use a scan tool in order to observe the HO2S voltage.

Is the HO2S voltage within the specified range?

350-550 mV

Go to Step 5

Go to Step 6

5
  1. The HO2S may be detecting a lean exhaust condition. Inspect for the following conditions:
    2. • Water intrusion into the HO2S connector
    • Exhaust leak between the HO2S and the engine
    • Vacuum leaks
    • Incorrect fuel pressure
    • Lean fuel injectors
    • Inaccurate MAF sensor
  2. Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 8

6
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector that contains the HO2S high signal circuit. Refer to Powertrain Control Module Replacement .
  3. Test the HO2S high signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 7

7

Test the HO2S high signal circuit for a short to the HO2S low signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 9

8

Inspect for poor connections at the harness connector of the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 10

9

Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

10

Replace the affected HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 or Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 .

Did you complete the replacement?

--

Go to Step 12

--

11

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 13

Go to Step 2

13

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK