P0138

CIRCUIT DESCRIPTION
The DTC P0138 Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 2 diagnostic monitors the output of the bank 1 post catalyst HO2S. The DTC P0158 Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 2 Sensor 2 diagnostic monitors the output of the bank 2 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. If the HO2S 2 voltage remains excessively high for an extended period of time, a DTC P0138 or DTC P0158 will be set.

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°F-230°F).
- Intake air temperature (IAT) is more than -14°C (+7°F) at engine start up.
- Vehicle speed is more than 56 km/h (35 mph) for 2 minutes with throttle position constant.

CONDITIONS FOR SETTING THE DTC
The maximum voltage is less than 350 mV.

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 is stored in the Freeze Frame buffer.

CONDITIONS FOR CLEARING THE MIL/DTC
- The MIL turns OFF after 3 consecutively passing trips without a fault present.
- A history DTC will clear 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
A DTC P0138 or DTC P0158 is more likely to set during city driving. Avoid operating the vehicle on the highway when validating a DTC P0138 or DTC P0158.

Check for any of the following conditions:
- If any DTCs other than fuel trim P0171, P0172, P0174, P0175, misfire P0300 - P0306, exhaust gas recirculation (EGR) P0400, and other HO2S are set, diagnosis those DTCs first.
- Before replacement of the HO2S, check the HO2S ground circuit for a possible open or short.
- The fuel pressure-The system will go rich if the fuel pressure is too high, and DTC P0138 or DTC P0158 may be set.
- Check the evaporative emission (EVAP) canister for fuel saturation.
- Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the fuel pressure regulator for the presence of fuel.
- A shorted HO2S-If the HO2S is internally shorted to voltage, the HO2S reading displayed on the scan tool will be more than 1 volt. Water contamination of the HO2S can cause a high HO2S voltage to be indicated. If DTC P0138 or DTC P0158 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. If a DTC P0138 or DTC P0158 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 (I/M) Heated Oxygen Sensor/Oxygen Sensor (HO2S/O2S) System Set Procedure. I/M HO2S/O2S System Set

An intermittent malfunction may be caused by a problem in the HO2S 2 electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions. Intermittent Conditions

If the DTC P0138 or DTC P0158 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

TEST DESCRIPTION

Step 1:

Steps 2-4:

Steps 5-7:

Steps 8-13:

The numbers below refer to the step numbers in the diagnostic table.
1. The Diagnostic System Check-Computers and Control Systems 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. This step determines if the fault is present. The vehicle must be at operating temperature.
3. This step checks whether the malfunction that caused the DTC P0138 or DTC P0158 is still present. 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 or DTC P0158 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 or DTC P0158 appears in the Not Ran Since Code Cleared list, the DTC P0138 or DTC P0158 diagnostic has not yet run. When a DTC P0138 or DTC P0158 does not appear in the Not Ran Since Code Cleared list, the DTC P0138 or DTC P0158 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, DTC P0138 or DTC P0158 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.
4. This step checks for an open in the HO2S 2 ground circuit.
5. This step checks for a short to voltage in the HO2S 2 signal circuit.
12. 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 or DTC P0158 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 or DTC P0158 appears in the Not Ran Since Code Cleared list, the DTC P0138 or DTC P0158 diagnostic has not yet run. When a DTC P0138 or DTC P0158 does not appear in the Not Ran Since Code Cleared list, the DTC P0138 or DTC P0158 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, DTC P0138 or DTC P0158 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.