GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2001 🢒 Seville - RHD 🢒 DTC 🢒 DTC P0131 or P0151

Circuit Description

The heated oxygen sensor (HO2S) is a sensor used to detect the oxygen content in the exhaust. The powertrain control module (PCM) supplies the sensor with a signal circuit and a ground circuit. The PCM supplies a bias voltage between the circuits. The sensor varies the voltage based on the oxygen content in the exhaust. When the exhaust stream is lean (more oxygen), the sensor produces a low voltage signal. When the exhaust stream is rich (less oxygen), the sensor produces a high voltage signal. The PCM monitors the HO2S signal during Closed Loop. To correct for rich or lean conditions, the PCM adjusts the injector pulse width. This diagnostic trouble code (DTC) indicates that the HO2S voltage remained too low for a calibrated period of time.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0201-P0208, P0410, P0440, P0442, P0443, P0446, P0449, P1415, P1416, or P1441 are not set.
    • The throttle position (TP) angle is between 3-25 percent.
    • The system voltage is between 9-18 volts.
    • The Closed Loop fuel control is enabled.
    • The air/fuel ratio is between 14.5-14.8.
    • The exhaust gas recirculation (EGR) Flow diagnostic test is not active.
    • No injectors are disabled.
    • The secondary air injection (AIR) diagnostic test is not active.
    • The catalyst diagnostic test is not active.
    • The conditions above are met for 3 seconds.

Conditions for Setting the DTC

The HO2S signal voltage remains below 75 mV (0.075 V) in Closed Loop mode.

OR

The HO2S signal voltage remains below 575 mV (0.575 V) during power enrichment (PE) mode.

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.

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

Notice: Do not solder heated oxygen sensor wires. Soldering the wires will result in the loss of the air reference to the sensor. Refer to Engine Electrical for proper wire and connection repair techniques.

Check for the following conditions:

    • The HO2S wire routing--The sensor wiring may be routed incorrectly and contacting the exhaust system.
    • The fuel pressure--Fuel pressure that is too low will cause a lean condition. The PCM will compensate for some decrease. However, if fuel pressure is low enough, the DTC may set.
    • The fuel injectors--If the fuel injectors deliver improper fuel volume at the specified pressure, the DTC may set. Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2 .
    • Exhaust leaks--Exhaust leaks may cause outside air to be drawn into the exhaust gas stream, and pass the HO2S sensor. This causes the HO2S signal to indicate a false lean condition.
    • Secondary air injection (AIR) system leaks--Check the AIR system hoses, the check valves, and the piping for leaks, for poor connections, or for signs of damage.
    • Vacuum leaks
        Check for the following:
       - Disconnected or damaged vacuum hoses.
       - Vacuum leaks at the throttle body, the exhaust gas recirculation (EGR) valve, the evaporative emission (EVAP) purge solenoid, and the intake manifold
    • Fuel contamination--Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust. Excessive alcohol in the fuel can cause a lean condition.
    • Check with the customer to verify the conditions during which the DTC set. Check to see if the vehicle ran out of fuel.

If none of the above conditions are present, replace the HO2S sensor. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 and Heated Oxygen Sensor Replacement - Bank 1 Sensor 2 .

Review the front and rear long term fuel trim values on the scan tool Failure Record data. If both banks were lean, check for a condition that would cause a lean engine operation. If only the rear bank was lean, the lean condition is isolated to that bank, such as an intermittent short to ground on the signal circuit.

If the condition is not present, check with the customer about the type of fuel used, for example over 10 percent ethanol fuel. Recommend using fuels of 10 percent or less ethanol.

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

  1. This step verifies whether the HO2S sensor voltage is switching from below 325 mV to above 625 mV, or remains below 300 mV.

    2.

  2. This step verifies the bias voltage from the PCM and verifies the condition of the circuit wiring. With the sensor disconnected, the scan tool should display a bias voltage near 450 mV.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. With the engine at operating temperature, operate the vehicle within the parameters specified in the Conditions for Running the DTC.
  2. Observe the affected HO2S voltage display on the scan tool.

Does voltage remain below the value specified?

75 mV

Go to Step 4

Go to Step 3

3
  1. Review and record the scan tool Failure Records Data.
  2. Operate the vehicle with the Failure Records conditions.
  3. With the scan tool, observe Specific DTC Information for this DTC until the test runs.

Does the scan tool indicate that the DTC passed?

--

Go to Intermittent Conditions

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect the affected HO2S connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the HO2S voltage display on the scan tool.

Does the scan tool display an HO2S voltage near the specified value?

450 mV

Go to Diagnostic Aids

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector C1. Refer to Powertrain Control Module Replacement .
  3. Test the affected HO2S sensor signal circuit for a short to chassis ground, or to the HO2S low circuit. Refer to Testing for Short to Ground in Wiring Systems.
  4. If a condition is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

6

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 7

--

7
  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 8

Go to Step 2

8

With a scan tool observe 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