P0138

DTC P0138

CIRCUIT DESCRIPTION
Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The engine control module (ECM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the ECM operates in Open Loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and Closed Loop is achieved, the HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream, and low HO2S voltage indicates a lean exhaust stream. If the ECM detects that HO2S 2 voltage stays above a specified value, DTC P0138 will set.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Sensor 2

CONDITIONS FOR RUNNING THE DTC
- DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0171, P0172, P0300, P0336, P0340, P0341, P0455, P0500, P0601, P0602, P0606, P0641, P0722, P0723 are not set.
- The Loop Status parameter is closed.
- The Air Fuel Ratio parameter is between 14.5:1 and 14.8:1.
- The transmission is in Drive with automatic transmission.
- DTC P0138 runs continuously when the above conditions are met.

CONDITIONS FOR SETTING THE DTC

Rich Test
- The ECM detects that the HO2S 2 voltage parameter is more than 1,065 mV.
- DTC P0138 sets during the rich test within 100 seconds when the above condition is met.

Decel Fuel Cutoff Test
- The ECM detects that the HO2S 2 voltage parameter is more than 587 mV.
- DTC P0138 sets during the decel fuel cutoff test within 10 seconds when the above condition is met.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

Step 1 - Step 9:

Step 10 - Step 16:

The number below refers to the step number on the diagnostic table.
2. If the voltage is more than the specified value, the condition is present.