For 1990-2009 cars

### 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 DCT 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

• | DTC P0141 and DTC P0161 are not set.

• | 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 more than 2,000 mV.

OR

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 Heated Oxygen Sensor/Oxygen Sensor 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 .

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

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. 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.

6. 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.

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 |

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 HO2S 2 voltage with a scan tool.

Does the HO2S 2 voltage switch below and above the specified value?

300-600 mV

Go to Step 3

Go to Step 4

3 |

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. Monitor the HO2S 2 voltage with a scan tool.
7. Operate the vehicle within Freeze Frame conditions as specified or until the DTC P0138 or DTC P0158 diagnostic test has run. Refer to the Test Description.

Does the HO2S 2 voltage switch below and above the specified value?

300-600 mV

Go to Diagnostic Aids

Go to Step 4

4 |

1. Disconnect the HO2S 2 electrical connector.
2. Turn ON the ignition, leaving the engine OFF.
3. Probe the harness side of the HO2S 2 ground circuit with a test lamp connected to B+.

Does the test lamp illuminate?

--

Go to Step 5

Go to Step 8

5 |

1. Turn OFF the ignition.
2. Disconnect the C2 electrical connector of the PCM.
3. Turn ON the ignition, leaving the engine OFF.
4. 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. 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 9

Go to Step 7

7 |

1. Inspect the HO2S signal circuit for the following conditions:

• | 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 12

Go to Step 11

8 |

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

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

9 | Replace the HO2S 2. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | -- | Go to Step 12 | --
10 |

1. Test the HO2S 2 signal circuit for a short to voltage.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 11

11 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 12 | --
12 |

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. Refer to the Test Description.

Does the DTC run and pass?

--

Go to Step 13

Go to Step 2

13 | 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