GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2003 🢒 Blazer - 2WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P0138

Circuit Description

Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The powertrain control module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S voltage that stays above a specified value, DTC P0138 will set.

Conditions for Running the DTC

Rich Test Enable:

    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0442, P0446, P0452, P0453, P0455, P0496 are not set.
    • The Loop Status parameter is closed.
    • The Ignition 1 Signal parameter is between 10-18 volts.
    • The Fuel Tank Level Remaining parameter is more than 10 percent.
    • The TP Sensor parameter is between 3-70 percent.

OR

Decel. Fuel Cutoff Test Enable:

    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0442, P0446, P0452, P0453, P0455, P0496 are not set.
    • The Loop Status parameter is closed.
    • The Ignition 1 Signal parameter is between 10-18 volts.
    • The Fuel Tank Level Remaining parameter is more than 10 percent.
    • The Engine Run Time parameter is more than 30 seconds.
    • The Decel. Fuel Cutoff parameter is active for more than 4 seconds.

Conditions for Setting the DTC

Rich Test

The PCM detects that the affected HO2S voltage parameter is more than 950 mV for 200 seconds.

or

Decel. Fuel Cutoff Test

The PCM detects that the affected HO2S voltage parameter is more than 250 mV for 5 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.

Test Description

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

  1. If the voltage does not change more that the specified value, the condition is present.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

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

Important: With the engine running, observe the HO2S Bank 1 Sensor 1 and HO2S Bank 2 Sensor 1 voltage parameters with a scan tool. The voltage should vary from below 300 mV to above 600 mV. If the voltage is not varying, refer to DTC P0131 or P0151 .

  1. Start the engine.
  2. Allow the engine to reach operating temperature. Refer to Scan Tool Data List .
  3. Operate the engine at 1,500 RPM for 30 seconds.
  4. While observing the affected HO2S voltage parameter with a scan tool, quickly cycle the throttle from closed throttle to wide open throttle, 3 times.

Did the HO2S voltage parameter change more than the specified value?

200 mV

Go to Step 3

Go to Step 4

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 affected HO2S.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter more than the specified value?

800 mV

Go to Step 6

Go to Step 5

5

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

Is the voltage more than the specified value?

2 V

Go to Step 7

Go to Step 8

6

Important: The sensor may be damaged if the circuit is shorted to a voltage source.

Test the HO2S high 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 17

Go to Step 14

7

Test the HO2S low 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 17

Go to Step 14

8
  1. Connect a 3-amp fused jumper wire between the high signal circuit of the HO2S harness connector on the engine harness side and a good ground.
  2. Observe the HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter less than the specified value?

100 mV

Go to Step 9

Go to Step 11

9
  1. Remove the jumper wire from the previous step.
  2. Connect a 3-amp fused jumper wire between the high signal circuit of the HO2S harness connector on the engine harness side and the low signal circuit of the HO2S harness connector on the engine harness side.
  3. Observe the HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter less than the specified value?

100 mV

Go to Step 12

Go to Step 10

10

Test the HO2S low signal circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 14

11

Test the HO2S high signal circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 14

12
  1. The HO2S may be detecting a rich exhaust condition or may be contaminated. Inspect for the following conditions:

    2. Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.

    • A silicon contaminated HO2S
    • Any water intrusion into the HO2S connector
    • Engine oil contaminated with fuel
    • An EVAP canister purge condition
    • An incorrect fuel pressure--Refer to Fuel System Diagnosis .
    • Any rich fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 .
    • A leaking fuel pressure regulator--Refer to Fuel System Diagnosis .
    • An inaccurate mass air flow (MAF) sensor--Refer to Scan Tool Data List .
    • An air intake restriction or collapsed air intake duct
  2. Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 13

13

Test for an intermittent and for a poor connection at 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 17

Go to Step 15

14

Test for an intermittent and for a poor connection at the PCM. 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 17

Go to Step 16

15

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

Did you complete the replacement?

--

Go to Step 17

--

16

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 17

--

17
  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 18

18

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK