GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM) supplies a voltage near 450 mV between the heated oxygen sensor (HO2S) high signal circuit and the low reference circuit. The HO2S varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean.

The ECM monitors and stores the HO2S voltage information. The ECM evaluates the HO2S voltage samples in order to determine the amount of time that the HO2S voltage was out of range. The ECM 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 HO2S utilizes the following circuits:

    • A signal circuit
    • A low reference circuit
    • An ignition 1 voltage circuit
    • A heater control circuit

The ECM monitors the HO2S voltage for being fixed above a predetermined voltage. If the ECM detects the voltage is too high, the DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0138 HO2S Circuit High Voltage Sensor 2

Conditions for Running the DTC

    • The engine is running.
    • The ignition 1 voltage is more 10 volts.
    • The DTC runs continuously once the above condition are met.

Conditions for Setting the DTC

The ECM detects that the HO2S voltage is more than 1200 mV for 5 seconds.

Action Taken When the DTC Sets

    • The malfunction indicator lamp (MIL) will illuminate after 3 consecutive trips with a fail.
    • The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records.
    • A history DTC is stored.

Conditions for Clearing the MIL/DTC

    • The MIL will turn OFF after 4 consecutive ignition cycles in which the diagnostic runs without a fault.
    • DTCs can be cleared by using the scan tool.
    • Disconnect the ECM battery feed for 10 seconds.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine 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 the engine.
  2. Allow the engine to reach the normal operating temperature.
  3. Operate the engine above 1,200 RPM for 30 seconds and then allow the engine to idle.
  4. Observe the heated oxygen sensor (HO2S) 2 voltage parameter with a scan tool.

Is the voltage more than the specified value?

900 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.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Intermittent Conditions

4

  1. Turn OFF the ignition.
  2. Disconnect the HO2S 2 harness connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the HO2S 2 voltage parameter with a scan tool.

Is the voltage more than the specified value?

900 mV

Go to Step 5

Go to Step 6

5

Test the HO2S 2 signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 8

6

Test or inspect for the following conditions:

    • HO2S harness connector for water intrusion.
    • High fuel system pressure-- Refer to Fuel System Diagnosis .
    • Rich fuel injectors or fuel injectors that are leaking--Refer to Fuel Injector Balance Test with Special Tool .
    • Fuel saturation of the evaporative emission (EVAP) canister
    • Exhaust system restrictions.
    • Leaking fuel pressure regulator. Inspect for evidence of fuel in the vacuum line--Refer to Fuel System Diagnosis .

Did you find and correct the condition?

--

Go to Step 11

Go to Step 7

7

Test for shorted terminals and 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 11

Go to Step 9

8

Test for shorted terminals and for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 10

9

Replace the HO2S. Refer to Heated Oxygen Sensor 2 Replacement .

Did you complete the replacement?

--

Go to Step 11

--

10

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 11

--

11

  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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 12

12

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK