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For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Tracker - 4WD 🢒 Engine 🢒 Engine Controls - 2.0L 🢒 DTC P0138

Circuit Description

The DTC P0138 Heated Oxygen Sensor (HO2S) Circuit Sensor 2 diagnostic monitors the output of the post catalyst HO2S. The powertrain control module (PCM) has the ability to determine the efficiency of the three-way catalytic (TWC) converter using the HO2S 1 and the HO2S 2. The HO2S produces a voltage between 100-900 mV that varies depending upon the oxygen content of the exhaust stream. The HO2S 1 output signal indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 output signal indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the effectiveness of the TWC converter. The HO2S 1 output signal is more active than the HO2S 2 output signal when the catalyst is operating efficiently.

Conditions for Running the DTC
    • Barometric pressure (BARO) is more than 75 kPa.
    • Engine coolant temperature (ECT) is between -8°C and +110°C (18-230°F).
    • Intake air temperature (IAT) is more than -8°C (18°F) at engine start up.
    • Vehicle is operating in Closed Loop.
    • Fuel level is more than 25%.

Conditions for Setting the DTC

The PCM detects an average signal voltage that exceeds a specified value.

Or

The lowest voltage produced by the HO2S 2 is above a specified value.

Action Taken When the DTC Sets
    • The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The PCM records the operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
    • The PCM turns OFF the MIL after 3 consecutive trips without a fault.
    • The PCM clears a history DTC after 40 consecutive warm-up cycles without a fault.
    • Perform the scan tool clear DTC Information function in order to clear the DTC.

Diagnostic Aids

Check for any of the following conditions:

    • If any DTCs other than fuel trim P0171, P0172, misfire P0300-P0304, exhaust gas recirculation (EGR) P0401, P0402, and other HO2S DTCs are set, diagnosis those DTCs first.
    • A misrouted heated oxygen sensor wiring. The sensor pigtail may be mispositioned and contacting the exhaust system.
    • Check for a shorted or open HO2S 2 signal or ground wire. Either of these conditions will cause a stuck high or low voltage reading on the scan tool.
    • Water contamination of the HO2S can cause an inaccurate HO2S voltage. Inspect whether DTC P0138 occurs when driving through standing water or when operating the vehicle in wet driving conditions. Replace the HO2S if the sensor is adversely affected by moisture.
    • A contaminated HO2S--Silicon contamination of the HO2S can cause an inaccurate HO2S voltage. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is found, determine the cause and correct the condition before replacing the HO2S.
    • Fuel contamination--Small amounts of water can be delivered to the fuel injectors and cause an inaccurate exhaust indication. Too much alcohol in the fuel can also cause an inaccurate exhaust indication. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

If a DTC P0138 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance Heated Oxygen Sensor/Oxygen Sensor System Set .

An intermittent malfunction may be caused by a fault in the HO2S sensor circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If the DTC P0138 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC first set.

Test Description

The numbers below refer to the step numbers in the diagnostic table.

  1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

    2.

  2. This step provides a quick check of the HO2S operation. The vehicle must be at operating temperature.

    3.

  3. This step checks whether the malfunction that caused the DTC is still present. If necessary, drive for 8 minutes between 50-60 km/h (30-40 mph), accelerate the vehicle to 100 kmh (60 mph) and then coast in fuel cutoff mode for an extended period of time. Monitor the DTC P0138 on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0138 appears in the Not Ran Since Code Cleared list, the diagnostic has not yet run.

  4. This step checks for a short to voltage on the HO2S 2 signal circuit.

  5. This step checks for an open in the HO2S 2 ground circuit.

    6.

  6. This step checks the HO2S 2 signal circuit. The DMM is used as a low current voltage source, taking the place of the HO2S. This tests the continuity of the signal wire , and if the PCM is able to receive and process a signal from the HO2S.

    7.

  7. A possible cause of a voltage short to the HO2S 2 signal circuit is water. Check the wiring harness and PCM connectors for water that may cause ignition positive voltage to be shorted to the signal circuit.

    8.

  8. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0138 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0138 appears in the Not Ran Since Code Cleared list, the DTC P0138 diagnostic has not yet run. When the DTC P0138 does not appear in the Not Ran Since Code Cleared list, the DTC P0138 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0138 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

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

Are any HO2S DTCs set other than DTC P0137 and DTC P0138?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Start the engine.
  2. Run the engine until the engine reaches a normal operating temperature.
  3. Increase the engine speed to 2,000 RPM for 1 minute.
  4. Monitor the applicable HO2S voltage with a scan tool.

Does the HO2S voltage switch below and above the specified range?

300-600 mV

Go to Step 4

Go to Step 5

4

  1. Turn ON the ignition, leaving the engine OFF.
  2. Perform the scan tool clear DTC information function.
  3. Operate the vehicle within Freeze Frame conditions as specified or until the applicable DTC diagnostic test has run. Refer to the Test Description.

Does the HO2S voltage switch below and above the specified range?

300-600 mV

Go to Diagnostic Aids

Go to Step 5

5

  1. Turn OFF the ignition.
  2. Disconnect the electrical connector of the suspect HO2S.
  3. Disconnect the C2 electrical connector of the PCM.
  4. Turn ON the ignition, leaving the engine OFF.
  5. Connect a DMM from ground to the oxygen sensor signal circuit on the harness side of the electrical connector.

Is the voltage at the specified value?

0.0 V

Go to Step 6

Go to Step 10

6

  1. Turn OFF the ignition.
  2. Connect the PCM C2 electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the HO2S ground circuit at the HO2S connector with a test lamp connected to B+.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 8

7

  1. Use the DMM and select the Millivolt DC scale.
  2. Connect one lead of the DMM to B+.
  3. Connect the other lead of the DMM to the signal circuit of the suspect HO2S.
  4. Start the engine.
  5. Observe the HO2S voltage parameter on the scan tool.

Is the voltage at the specified value?

1,000 mV

Go to Step 11

Go to Step 9

8
  1. Check for an open in the ground circuit of the HO2S.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 12

9
  1. Check the HO2S signal circuit for the following conditions:
    2. • Open Circuit
    • Short to ground
    • High circuit resistance
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 12

10

Repair the short to voltage in the HO2S signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 13

--

11

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

Did you complete the replacement?

--

Go to Step 13

--

12

Important: The replacement PCM must be programmed.

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 13

--

13

  1. Turn OFF the ignition.
  2. Remove the DOME fuse for 30 seconds.
  3. Install the DOME fuse.
  4. Reset the radio clock time. Refer to Radio/Audio System Description and Operation in Entertainment.
  5. Start the engine.
  6. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0138 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 14

Go to Step 2

14

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