GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The heated oxygen sensor (HO2S) produces a voltage that varies between 100-900 mV under normal operating conditions. The engine control module (ECM) produces a bias voltage on the HO2S signal circuit of 420-480 mV. The reference ground for the sensor is provided through the ECM. The ECM monitors the signal voltage to determine if the exhaust is lean or rich. The oxygen sensor voltage is high when the exhaust is rich, and low when the exhaust is lean. The ECM constantly monitors the HO2S signal during the Closed Loop operation. If the ECM detects an HO2S voltage that stays above a specified value, DTC P0138 will set for bank 1 sensor 2, or DTC P0158 will set for bank 2 sensor 2.

Conditions for Running the DTC

    • DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0131, P0132, P0133, P0135, P0137, P0139, P0140, P0141, P0151, P0152, P0153, P0155, P0157, P0159, P0160, P0161, P1137, P1138, P1157, P1158 P2231, P2234, P2237, P2240, P2243, P2247, P2251, P2254, P2626, P2629 are not set.
    • The engine is running.

Conditions for Setting the DTC

The ECM detects that the HO2S parameter is more than 2 volts for longer than 1.6 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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

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

1

Did you perform the Diagnostic System Check-Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Allow the engine to reach normal operating temperature. Refer to Scan Tool Data List .
  2. Observe the HO2S Bank 1 Sensor 2 or HO2S Bank 2 Sensor 2 parameter with the scan tool.

Is the voltage more than the specified value?

1,000 mV

Go to Step 4

Go to Step 3

3

  1. Observe the Freeze Frame/Failure Records 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. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4

  1. Turn OFF the ignition.
  2. Disconnect the heated oxygen sensor (HO2S).
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage between the HO2S high signal circuit, on the engine harness side connector, and a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage more than the specified value?

475 mV

Go to Step 5

Go to Step 8

5

Measure the voltage from the HO2S low signal circuit on the engine harness side connector to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage less than the specified value?

50 mV

Go to Step 6

Go to Step 7

6

Test the HO2S high signal circuit for a short to voltage or a short to the HO2S heater low control circuit. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

7

Test the HO2S low signal circuit for a short to voltage. Refer to Circuit Testing , Wiring Repairs , and Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

8

The HO2S may be detecting a rich exhaust condition or may be contaminated. Inspect for one of the following conditions:

    • HO2S connector water intrusion
    • A silicon-contaminated HO2S
    • Fuel-contaminated engine oil
    • Incorrect fuel pressure--Refer to Fuel System Diagnosis .
    • Rich fuel injectors

Repair any of the above or similar engine conditions, as necessary.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 9

9

Test for poor connections and shorted terminals at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections , Connector Repairs , and Heated Oxygen Sensor Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 11

10

Inspect for poor connections and shorted terminals at the engine control module (ECM). 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 13

Go to Step 12

11

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

Did you complete the replacement?

--

Go to Step 13

--

12

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 13

--

13

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 14

14

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK