GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Safari - 4WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P0131 or P0151

Circuit Description

The heated oxygen sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S heater quickly warms the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S must reach a high temperature to produce a voltage. When the HO2S reaches operating temperature, the control module monitors the HO2S bias, or reference, voltage. The control module also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or lean out the mixture, when the HO2S detects a rich exhaust condition.

This DTC is designed to detect an HO2S voltage that remains at a low voltage for more than a specified number of seconds during the test conditions.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0440, P0442, P0446, P0452, P0453, or P1441 are not set.
    • The ignition 1 signal is between 9-18 volts.
    • The fuel tank level remaining is more than 10 percent.
    • The loop status is Closed.
    • Intrusive tests are not in progress.
    • The scan tool output controls are not active.

Lean Test Enable

    • The air fuel ratio is between 14.5:1-14.7:1.
    • The throttle position (TP) is between 3-70 percent.

Power Enrichment Test Enable

    • The power enrichment is active.
    • The above condition is met for 2 seconds.

Conditions for Setting the DTC

Lean Test

The HO2S voltage is less than 99 mV for 99 seconds.

Power Enrichment Test

The HO2S voltage is less than 600 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.

Test Description

The number below refers to the step number on the diagnostic table.

  1. Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor, inspect for lean conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S needs replacement or a lean condition exists only on the suspect HO2S cylinder bank.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2
  1. Run the engine until the engine reaches a normal operating temperature.
  2. Using the scan tool, monitor the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage fixed at less than the specified value?

99 mV

Go to Step 4

Go to Step 3

3

The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:

    • A HO2S high signal wire intermittently shorted to engine ground or to the HO2S low reference circuit
    • Any water intrusion into the HO2S connector
    • An exhaust leak between the HO2S and the engine Refer to Exhaust Leakage in Engine Exhaust.
    • An intermittent lean engine condition such as the following:
       - A vacuum leak
       - An incorrect or defective positive crankcase ventilation (PCV) valve or system
       - An incorrect fuel pressure
       - Any lean fuel injectors
       - An inaccurate MAF sensor
       - A lean engine condition during power enrichment operation
    • Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

--

Go to Step 10

Go to Intermittent Conditions

4
  1. Turn the ignition OFF.
  2. Disconnect the HO2S connector for the sensor that applies to this DTC.
  3. Turn ON the ignition, with the engine OFF.
  4. Using the scan tool, monitor the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage within the specified range?

351-551 mV

Go to Step 5

Go to Step 6

5

  1. The HO2S may be detecting a lean exhaust condition. Inspect for one of the following conditions:
    2. • Any water intrusion into the HO2S connector
    • An exhaust leak between the HO2S and the engine.
    • Refer to Exhaust Leakage in Engine Exhaust.
    • Any vacuum leaks
    • An incorrect fuel pressure
    • Any lean fuel injectors
    • An 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 10

Go to Step 8

6
  1. Turn the ignition OFF.
  2. Disconnect the PCM connector that contains the HO2S high signal circuit.
  3. Using a DMM, measure and record the resistance between the HO2S high signal circuit and ground.
  4. Measure and record the resistance between the HO2S high signal circuit and the HO2S low reference circuit.

Is the resistance infinite on both circuits?

∞

Go to Step 9

Go to Step 7

7

Repair the short to ground or short to the HO2S low reference circuit in the HO2S high signal circuit . Refer to Wiring Repairs in Wiring Systems.

Did you complete the action?

--

Go to Step 10

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8

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

Did you complete the action?

--

Go to Step 10

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9

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the action?

--

Go to Step 10

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10
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the engine 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 11

Go to Step 2

11

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