P0138

CIRCUIT DESCRIPTION
The rear heated oxygen sensor (HO2S) 2, located in the exhaust stream past the 3-way catalytic converter (TWC), produces an output signal relative to the oxygen storage capacity of the TWC. The powertrain control module (PCM) then calculates the ability of the TWC to convert the exhaust emissions effectively. Whenever the catalytic converter is functioning properly, the rear HO2S 2 signal will be far less active than the signal produced by the front oxygen sensor 1 (O2S) 1.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0169, P0171, P0172, P0178, P0179, P0201-P0204, P0300, P0301-P0304, P0336, P0440, P0446, P0452, P0453, P0506, P0507, P0601, P0602, or P1441 are not set.
^ The engine coolant temperature (ECT) is more than 70°C (158°F).
^ The engine run time is more than 10 seconds.
^ The throttle position (TP) angle is between 8-50 percent.
^ The above conditions are met for 3.8 seconds.
^ Closed Loop operation.
^ The system voltage is more than 10 volts.
^ The fuel level is more than 10 percent.
^ The fuel ethanol composition less than 88 percent.
^ The manifold absolute pressure (MAP) is greater than 25 kPa.

CONDITIONS FOR SETTING THE DTC
The HO2S 2 voltage is more than 1042 mV for 50 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: Never solder the HO2S wires. For proper wire and connector repair refer to Wiring Repairs or Connector Repairs in Diagnostic Aids.

Inspect the following items:
^ The fuel pressure-If the pressure is too high, the system will run rich. The PCM can compensate for some increase. However, if the pressure gets too high, the DTC P0138 may set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ A rich injector-Perform an Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 or Fuel Injector Balance Test with Special Tool. Component Tests and General Diagnostics Component Tests and General Diagnostics
^ A leaking injector-Refer to Fuel Injector Balance Test with Tech 2 or Fuel Injector Balance Test with Special Tool. Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Fuel contaminated oil
^ The TP sensor-An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving.
^ False rich indication due to silicon contamination of the HO2S-This diagnostic trouble code (DTC), accompanied by a lean driveability condition and a powdery white deposit on the sensor, may indicate a false rich indication.

An intermittent may be caused by any of the following conditions:
^ A poor connection
^ Rubbed through wire insulation
^ A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections Diagnostic Aids.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagnostic Aids.

TEST DESCRIPTION

Steps 1-6:

Steps 7-11:

The numbers below refer to the step numbers on the diagnostic table.
2. In order to determine if the engine is rich during de-acceleration fuel cut-off (DFCO) operate the vehicle up to highway speed conditions and release the accelerator pedal, allowing the vehicle to coast in gear. Monitor the scan tool HO2S voltage and the DFCO parameter. A rich condition will cause the HO2S voltage to be more than 468 mV during DFCO.
5. Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor, check for rich conditions that would affect both cylinder banks. An opposite bank sensor With normal HO2S voltage activity indicates the suspect HO2S is defective, or a rich condition exists only on the suspect HO2S cylinder bank.
6. This step checks the HO2S HIGH signal circuit for a short to voltage.
8. An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of unapproved silicon RTV engine gasket material or the use of silicon-based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also become contaminated.