P0138

Heated Oxygen Sensor 2 (HO2S 2) Circuit:

Circuit Description
The Powertrain Control Module (PCM) has the ability to monitor this process by using a rear Heated Oxygen Sensor (HO2S 2). The rear HO2S 2, located in the exhaust stream past the catalytic converter produces an output signal relative to the oxygen storage capacity of the catalytic converter. The PCM can than calculate the catalytic converter's ability to convert exhaust emissions effectively. Whenever the catalytic converter is functioning properly, the rear HO2S 2 signal will be far less active than the signal produced by the front Oxygen Sensor 1(O2S 1).

Conditions for Setting the DTC
- The Throttle Position angle is between 8% and 51%
- The Air Fuel Ratio is between 14.5:1 and 14.7:1
- The engine coolant temperature is greater than 70°C (158°F)
- The vehicle is in "Closed Loop"
- The above conditions are met for greater than 10 seconds
- The rear HO2S 2 remains greater than 1042 mV 93 seconds
- The engine run time is greater than 25 seconds
- When one or more of the following DTCs are not set P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0300-P0304, P0506, P0507, PO1406, P1441.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
- The PCM 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 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.
- DTCs can be cleared by using a scan tool.

NOTE: 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 oxygen sensor's performance.

Diagnostic Aids
Check for:
- Rich exhaust - An overly rich exhaust may load the catalytic converter causing high rear HO2S 2 signal voltages.
- Silicone contamination - A false rich condition may be caused by silicone contamination of the rear HO2S 2. This will be indicated by a powdery white deposit on the rear HO2S 2.
- Faulty HO2S 2 - Whenever the rear HO2S 2 is internally shorted, the rear HO2S 2 voltage displayed on a scan tool will be over 1 volt. Disconnect the rear HO2S 2. Jumper the rear HO2S low circuit to engine ground. Whenever the rear HO2S 2 voltage goes from 100 mV to around 450 mV, replace the rear HO2S 2.
- Intermittent test - Observe the rear HO2S 2 on a scan tool while moving related connectors and wiring harness with the key "ON." Whenever the fault is induced, the rear HO2S 2 display will change. This may help isolate the location of the malfunction.
- Damaged harness - Inspect the PCM harness for a short to voltage in the rear HO2S 2 low circuit.

DTC P0138 - Heated Oxygen Sensor (HO2S) Circuit High Voltage Sensor 2:

Test Description
Numbers below refer to the step numbers on the Diagnostic Table.
1. The OBD System Check prompts the Technician to complete some basic checks and store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. This information on the scan tool can be referred to later.
2. This step determines if DTC P0138 is the result of a hard fault or an intermittent condition.
5. Disconnecting the rear HO2S 2 and jumpering the rear HO2S 2 low and the rear HO2S 2 high to ground should cause the scan tool to display the rear HO2S 2 voltage below 0.2 volt (200 mV). When the signal voltage remains high, then the PCM is faulty.
8. Before replacing the PCM, check terminals for improper mating, broken locks, or physical damage to the wiring harness. Replacement PCM must be reprogrammed. Refer to the latest Tech line information for programming procedures.