P0138
P0138 O2 Bank 1 Sensor 2 High Voltage
Circuit Description
In order to control emissions, a catalytic converter is used to convert harmful emissions into harmless water vapor and carbon dioxide.
The powertrain control module (PCM)/engine control module (ECM) has the ability to monitor this process by using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Oxygen Sensor (O2S).
If the HO2S pigtail wiring, connector or terminal are damaged, the entire HO2S assembly must be replaced. Do not attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the HO2S wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the air reference and degrade HO2S performance.
Conditions for Setting the DTC
- HO2S 2 Voltage is greater than 0.952 volt or greater than 0.478 volt in Decel Fuel Cutoff (DFCO) mode.
- Closed loop stoichiometry.
- Engine Coolant Temperature (ECT) is greater than 140 °F (60 °C).
- Air/fuel ratio is between 14.5:1 and 14.8:1.
- Throttle Position (TP) Sensor is between 5% and 50%.
- 2 second delay after conditions are met.
- No related malfunctions (see P0131).
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The PCM/ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history Diagnostic Trouble Code (DTC) is stored.
Conditions for Clearing the MIL/DTC
- The MIL will turn off after three 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.
- DTC(s) can be cleared by using the scan tool.
Diagnostic Aids
- Rich exhaust - An overly rich exhaust may load the catalyst causing high HO2S signal voltages.
- Silicone contamination - A false rich condition may be caused by silicone contamination of HO2S. This will be indicated by a powdery white deposit on the sensor.
- Faulty HO2S - If HO2S is internally shorted, the HO2S voltage displayed on a scan tool will be over 1 volt. Disconnect the HO2S and jumper the sensor low circuit to engine ground; if the displayed voltage goes from over 1000 mV to around 450 mV, replace the HO2S.
- Intermittent test - Observe HO2S on Tech 1 while moving related connectors and wiring harness with the key ON. If the failure is induced, the HO2S display will change. This may help isolate the location of the malfunction.
Step 1 - 4:
Step 5 - 10:
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool it applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
2. This step determines if DTC P0138 is the result of a hard failure or an intermittent condition.
5. Disconnecting the HO2S and jumpering the sensor signal circuit and the sensor low circuit to ground should cause the scan tool to display HO2S voltage below 100 mV (0.1 V). If the signal voltage is still high, the PCM/ECM is malfunctioning.
8. The replacement PCM/ECM must be reprogrammed. Refer to the latest Techline procedure for PCM/ECM reprogramming.