P0138

Diagnostic Chart:

Engine Controls Schematics: HO2S #1 and HO2S #2:

CIRCUIT DESCRIPTION
To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three way catalytic converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the HO2S 2 signal voltage remains excessively high for an extended period of time, DTC P0138 will be set.

CONDITIONS FOR RUNNING THE DTC
^ No active TP, MAP, MAF, ECT, IAT Sensor, fuel injector circuit, EVAP, AIR DTCs present.
^ System voltage between 9.0 and 18.0 volts.
^ Throttle angle between 5% and 40%.
^ Closed loop commanded air/fuel ratio is between 13.0 and 16.5.
^ Commanded AIR pump OFF.

CONDITIONS FOR SETTING THE DTC
HO2S 2 signal voltage remains above 999 mV during normal closed loop operation for greater than 5 minutes.
OR
HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation for greater than 100 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.
^ If the misfire is determined to be catalyst damaging, the PCM will flash the MIL.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
^ The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

DIAGNOSTIC AIDS
Inspect for the following conditions:
^ Fuel pressure. An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Rich injectors. Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ Leaking injector. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Evaporative Emissions (EVAP) canister purge. Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to EVAP Control System Diagnosis (General Table).
^ MAF sensor. Disconnect the MAF sensor and see if rich condition is corrected. If so, replace MAF sensor. Refer to MAF Sensor Replacement.
^ Test for a leaking fuel pressure regulator diaphragm by inspecting the vacuum line to regulator for the presence of fuel.
^ TP sensor. An intermittent TP sensor output will cause the system to go rich, due to a false indication of the engine accelerating.
^ Shorted Heated Oxygen Sensor (HO2S). If the HO2S is internally shorted the HO2S voltage displayed on the scan tool will be over 1.0 volts. Try disconnecting the affected HO2S and jumper the HO2S low circuit to ground with the key ON, engine OFF. If the displayed HO2S voltage changes from over 1000 mV to around 450 mV, replace the HO2S. Silicon contamination of the HO2S can also cause a high HO2S voltage to be indicated. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is evident, replace the affected HO2S.
^ Open HO2S Signal or Low Circuit or Faulty HO2S. A poor connection or open in the HO2S signal or low circuit can cause the DTC to set during deceleration fuel mode. An HO2S which is faulty and not allowing a full voltage swing between the rich and lean thresholds can also cause this condition. Operate the vehicle while monitoring the HO2S voltage with a scan tool. If the HO2S voltage is limited within a range between 300 mV to 600 mV, check the HO2S signal and low circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the HO2S.

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams for diagnosis and repair.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Diagrams for diagnosis and repair.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
The number below refers to the step number on the diagnostic table:
3. DTC P0138 being set during deceleration fuel mode operation may indicate a condition described in the Diagnostic Aids above. If the DTC P0138 test passes while the Failure Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
7. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.