P0138 or P0158 (Federal W/RPO L35)

CIRCUIT DESCRIPTION
The heated oxygen sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater that is necessary in order to quickly warm the sensor to the operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches the operating temperature, the control module monitors the HO2S bias, or reference, voltage. The control module also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or lean out the mixture, when the HO2S detects a rich exhaust condition.

This DTC is designed to detect an HO2S voltage that remains high for more than a specified number of seconds during test conditions.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0401, P0404, P0405, P0440, P0442, P0446, P0452, P0453, P1404, or P1441 are not set.
^ The ignition 1 signal is between 9-18 volts.
^ The fuel tank level remaining is more than 10 percent.
^ The loop status is closed.
^ Intrusive tests are not in progress.
^ Scan tool output controls are not active.

Rich Test Enable
^ The air fuel ratio is between 14.5:1-14.7:1.
^ The throttle position (TP) is between 3-50 percent.

Decel Fuel Cut-off Test Enable
^ Decel fuel cut-off (DFCO) is active.
^ The above condition is met for 3 seconds.

CONDITIONS FOR SETTING THE DTC

Rich Test
The HO2S voltage is more than 994 mV for 450 seconds.

Decel Fuel Cut-off Test
The HO2S voltage is more than 469 mV for 10 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.

DIAGNOSTIC AIDS

IMPORTANT: Before you service the PCM, remove any debris from the PCM connector surfaces. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

^ Check the HO2S electrical connections for evidence of water intrusion - Water in the connector causes the B+ supply to the heater to bleed over to the signal circuit.
^ Check the fuel pressure - The system goes rich if the pressure is too high. The PCM compensates for some increase. If the fuel pressure is too high, a DTC may set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Check for rich injectors - Perform the Injector Balance Test. Component Tests and General Diagnostics
^ Check for a leaking injector.
^ Check the fuel pressure regulator - Inspect the vacuum line to the fuel pressure regulator for evidence of fuel. Refer to the Fuel System Diagnosis. Component Tests and General Diagnostics
^ Check the evaporative emissions (EVAP) canister purge - Inspect the canister for fuel saturation. If the canister is full of fuel, check the canister control and the hoses. Refer to Evaporative Emission (EVAP) Control System Description.
^ Check the mass airflow (MAF) sensor- Disconnect the MAF sensor and see if the rich condition is corrected. If the condition is corrected, check for proper installation. If the MAF sensor is installed correctly, replace the MAF sensor. If the MAF sensor is installed backwards, the system goes rich. The plastic portion of the sensor has arrows that indicate the correct airflow direction. The arrows must point toward the engine.
^ Check the HO2S oxygen supply - An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provides the supply of oxygen. Inspect the HO2S wires and the connections for breaks or for contamination. Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagnostic Aids.
^ Check the TP sensor - An intermittent TP sensor output causes the system to go rich, due to a false indication of the engine accelerating. For an intermittent condition, refer to Symptoms - Computers and Controls Systems. Symptom Related Diagnostic Procedures

TEST DESCRIPTION

DTC P0138 Or P0158 (Federal W/RPO L35) - Steps 1-6:

DTC P0138 Or P0158 (Federal W/RPO L35) - Steps 7-13:

The numbers below refer to the step numbers on the diagnostic table.
2. If the voltage is above the specified value the condition is present.
5. Jumpering the HO2S low signal circuit to ground is necessary for the PCM to correctly measure the voltage on the HO2S high signal circuit. If the HO2S voltage is within the specified range, the PCM and HO2S high signal circuit are OK.

For a description of the RPO Code(s) shown in this article or any of the images therein, refer to the RPO Code List found at Vehicle/Application ID". RPO Codes