P0138
DTC P0138 HO2S Circuit High Voltage Sensor 2
Circuit Description
The HO2S 2, located in the exhaust stream past the catalytic converter, produces an output signal proportional to the oxygen storage capacity of the catalytic converter. The PCM uses this signal to measure the ability of the catalytic converter to convert the exhaust emissions effectively. When the catalytic converter is functioning properly, the HO2S 2 signal is much less active than the signal produced by the front oxygen sensor 1 (O2S 1).
Conditions for Running the DTC
- There are no active manifold absolute pressure (MAP) DTCs.
- There are no active intake air temperature (IAT) DTCs.
- There are no active engine coolant temperature (ECT) DTCs.
- There are no active throttle position (TP) DTCs.
- There are no active fuel trim DTCs.
- There are no active injector control DTCs.
- There are no active misfire DTCs.
- There are no active crankshaft position (CKP) DTCs.
- There are no active evaporative emissions (EVAP) DTCs.
- There are no active idle air control (IAC) DTCs.
- There are no active PCM memory DTCs.
- The air flow is more than 5.5 grams per second.
- The engine coolant temperature (ECT) is more than 40° C (104° F).
- The throttle position (TP) angle is between 5 - 50 percent for 4 seconds.
- The above conditions are met for 3.8 seconds.
Conditions for Setting the DTC
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) if a failure is detected during 2 consecutive ignition cycles.
- The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is recorded in the scan tool's Freeze Frame and Failure Records.
Conditions for Clearing the MIL/DTC
- The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm up cycles without a fault.
- The DTCs can be cleared using the scan tool Clear Information function.
Diagnostic Aids
Whenever the oxygen sensor pigtail wiring, connector or terminal are damaged, the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, the O2S must have a clean air reference. This clean air reference is obtained by way of the oxygen sensor wires Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference and degrade the performance of the oxygen sensor.
Check for the following conditions.
- A rich exhaust - An overly rich exhaust may load the catalytic converter causing high rear HO2S 2 signal voltages.
- Silicone contamination - Inspect the rear HO2S 2 for a white powdery coating. Contamination of the HO2S 2 may result in a high, but false, voltage signal, which will be interpreted by the PCM as a rich exhaust.
- A faulty HO2S 2 - Whenever the rear HO2S 2 is internally shorted, the rear HO2S 2 voltage displayed on a scan tool will be more than 1.0 volt. Disconnect the rear HO2S 2 Jumper the rear HO2S low circuit to the engine ground. If the rear HO2S 2 voltage increases from 100 mV to approximately 450 mV, replace the rear HO2S 2.
- An intermittent test - With the ignition ON, observe the rear HO2S 2 on the scan tool while moving any related connectors and wiring harnesses. If the fault is induced, the rear HO2S 2 display will change. This may help isolate the location of the malfunction.
- A damaged harness - Inspect the PCM harness for a short to voltage in the rear HO2S 2 low circuit.
Steps 1 - 3:
Steps 4 - 5:
Steps 6 - 9:
Steps 10 - 11:
Test Description
The number below refers to the step number 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 HO2S voltage to be above 468 mV during DFCO.
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 un-approved 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 get contaminated.
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 un-approved 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 get contaminated.