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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Safari - 4WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P0137

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 contains a heater that is necessary in order to quickly warm the sensor to the operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order 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 low 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, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0401, P0404, P0405, P0410, P0440, P0442, P0446, P0452, P0453, P1120, P1125, P1220, P1221, P1258, P1404, P1415, P1416, P1514, P1515, P1516, P1517, or P1518 are not set.
    • The air fuel ratio is between 14.5:1 and 14.7:1.
    • The throttle position (TP) is between 3-70 percent.
    • The loop status is closed.
    • The ignition 1 signal is between 9-18 volts.
    • The fuel tank level remaining is more than 10 percent.
    • Intrusive tests are not in progress.
    • Scan tool output controls are not active.

Conditions for Setting the DTC

The HO2S voltage is less than 26 mV for 750 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.

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: Never solder the HO2S wires. For the proper wire and connector repair procedures, refer to Wiring Repairs in Wiring Systems.

Check for the following conditions:

    • A sensor pigtail may be mispositioned and contacting the exhaust system
    • An intermittent ground in the wire between the connector and the sensor
    • A poor powertrain control module (PCM) ground
    • Any lean injectors--Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 .
    • A contaminated fuel supply--Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can also set this DTC. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • The fuel pressure--If the pressure is too low, the system will be lean. In order to confirm a low fuel pressure, monitor a fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Diagnosis .
    • Any exhaust leaks--If there is an exhaust leak the engine may pull outside air into the exhaust and past the sensor. Refer to Exhaust Leakage in Engine Exhaust.
    • Any vacuum or crankcase leaks can cause a lean condition or a high idle.

Any of the following conditions can cause an intermittent:

    • A poor connection
    • Rubbed-through wire insulation
    • A broken wire inside the insulation

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 check 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?

--

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?

26 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 an 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 (PCV) valve or system
       - An incorrect fuel pressure
       - Any lean fuel injectors
       - An inaccurate mass air flow (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 Diagnostic Aids

4
  1. Turn the ignition OFF.
  2. Disconnect the HO2S connector for the sensor that applies to this DTC.
  3. Turn the ignition ON, leaving 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. Check 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 mass air flow (MAF) sensor
  2. Repair any of the above or similar engine conditions as necessary.

Did you complete the action?

--

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 tests?

∞

Go to Step 9

Go to Step 7

7

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

Did you complete the action?

--

Go to Step 10

--

8

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

Did you complete the action?

--

Go to Step 10

--

9

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the action?

--

Go to Step 10

--

10
  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 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