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 Bank 1 HO2S 2 and the Bank 1 HO2S 3 heated oxygen sensors. The Bank 1 HO2S 2 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 2 signal will be far more active than that produced by the Bank 1 HO2S 3 sensor. If the Bank 1 HO2S 2 signal voltage remains excessively high for an extended period of time, DTC P0138 will be set.
Conditions for Setting the DTC
| • | Engine running in closed loop |
| • | Air/Fuel ratio between 14.5 to 1 and 14.8 to 1. |
| • | No active transmission, TP sensor, EVAP system, EGR, misfire, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, EVAP solenoid ODM, ECT sensor, or MAF sensor DTC(s) present. |
| • | Bank 1 HO2S 2 signal voltage remains above 999 mV during normal closed loop operation. |
OR
Bank 1 HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation.
Either condition must be present for at least 5 seconds.
Action Taken When the DTC Sets
| • | The PCM will illuminate the 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 Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF 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 the scan tool Clear Information function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Check for the following conditions:
| • | Fuel pressure high - An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Pressure Test . |
| • | Fuel injector(s) contaminated or faulty - Refer to Fuel Injector Balance Test . |
| • | Check the EVAP Canister for Fuel Saturation. If the EVAP Canister is full of fuel, check canister control and hoses. Refer to Evaporative Emission Control System Diagnosis . |
| • | Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace the MAF sensor. |
| • | Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the fuel pressure regulator for the presence of fuel. |
| • | An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. |
| • | Shorted (HO2S) - If the HO2S is internally shorted, the HO2S voltage displayed on the scan tool will be greater than1 volt. Try disconnecting the affected HO2S and jumper the HO2S low circuit to ground with the ignition key ON. If the displayed HO2S voltage changes from over 1000mV to around 450mV, 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 300mV to 600mV, check the HO2S signal and low circuit wiring and associated terminal connections . If the wiring and connections are OK, replace the HO2S. |
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
-
DTC P0138 being set during deceleration fuel mode operation (vehicle speed above 25 mph, TP angle below 3%) may indicate a condition described in the Diagnostic Aids above. If the DTC P0138 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Fail 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.
-
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. Refer to PCM Replacement and Programming Procedures in Powertrain Control Module (PCM) and Sensors.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 |
Does the Bank 1 HO2S 2 voltage remain above the specified value? | 999 mV | ||
Does scan tool indicate DTC P0138 failed this ignition? | -- | Refer to Diagnostic Aids | ||
4 |
Does scan tool indicate Bank 1 HO2S 2 voltage near the specified value? | 450 mV | Refer to Diagnostic Aids | |
5 |
Does DVM indicate a voltage above specified value? | 999 mV | ||
6 | Repair short to voltage in the Bank 1 HO2S 2 signal circuit. Refer to Heated Oxygen Sensor (HO2S) Repair in Electrical Diagnosis. Is the action complete? | -- | -- | |
Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||
8 |
Does scan tool indicate DTC P0138 failed this ignition? | -- | System OK |
