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 Setting the DTC
| • | No active misfire, fuel injector circuit, TP sensor, EVAP, IAT sensor, MAP sensor, Fuel trim, EGR, ECT sensor, MAF sensor or CKP sensor DTCs present. |
| • | Throttle angle between 3% and 40%. |
| • | Ignition voltage is more than 9.0 volts. |
| • | Closed loop commanded air/fuel ratio is between 14.5 and 14.8. |
| • | HO2S 2 signal voltage remains above 999mV during normal closed loop operation for 224 seconds. |
OR
HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation for 360 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. |
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 the scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Fuel pressure. An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Diagnosis . |
| • | Rich injector(s). Refer to Fuel Injector Balance Test . |
| • | Leaking injector. Refer to Fuel System Diagnosis . |
| • | Evaporative emissions (EVAP) canister purge. Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to Evaporative Emission Control System Diagnosis . |
| • | MAF sensor. Disconnect the MAF sensor and see if rich condition is corrected. If so, replace MAF sensor. Refer to MAF Sensor Replacement . |
| • | Check for a leaking fuel pressure regulator diaphragm by checking 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 volt. 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. Refer to Heated Oxygen Sensor (HO2S) Replacement . |
| • | 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. Refer to Heated Oxygen Sensor (HO2S) Replacement . |
Test Description
The number below refers to the step number on the diagnostic table:
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain OBD System Check performed? | -- | ||
2 |
Does the HO2S 2 voltage remain above the specified value? | 800 mV | ||
Does scan tool indicate DTC P0138 failed this ignition? | -- | Go to Diagnostic Aids | ||
4 | Disconnect HO2S 2 and jumper HO2S low circuit to ground. Does scan tool indicate HO2S 2 voltage near the specified value? | 450 mV | Go to Diagnostic Aids | |
5 |
Does DMM indicate any voltage? | -- | ||
6 | Repair short to voltage in the HO2S 2 signal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
7 |
Important: Replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | |
8 |
Does scan tool indicate DTC P0138 failed this ignition? | -- | System OK |
