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 powertrain control module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started, the PCM 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. Low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S 2 voltage that 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

Rich Test Enable
* DTCs P0068, P0069, P0097, P0098, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0121, P0201, P0202, P0203, P0204, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0461, P0496, P1516, P1681, P2101, P2135, or P2176 are not set.
* The Loop Status parameter is closed.
* The Ignition 1 Signal parameter is between 11-18 volts.
* The Fuel Level Sensor parameter is more than 10 percent.
* The Throttle Position (TP) Sensor parameter is between 15-50 percent.
* The catalyst diagnostic is not active.
* Traction control is not active.
* The air/fuel ratio is between 14.5-14.8.
* DTC P0138 runs continuously during the rich test when the above conditions have been met for 2 seconds.

Decel Fuel Cutoff Test Enable
* DTCs P0068, P0069, P0097, P0098, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0121, P0201, P0202, P0203, P0204, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0461, P0496, P1516, P1681, P2101, P2135, or P2176 are not set.
* The Loop Status parameter is closed.
* The Ignition 1 Signal parameter is between 11-18 volts.
* The Fuel Level Sensor parameter is more than 10 percent.
* The Engine Run Time parameter is more than 200 seconds.
* The Decel Fuel Cutoff is active.
* The catalyst diagnostic is not active.
* Traction control is not active.
* DTC P0138 runs continuously during the decel fuel cutoff test when the above conditions have been met for 2 seconds.

Conditions for Setting the DTC

Rich Test
* The PCM detects that the HO2S 2 parameter is more than 1,000 mV.
* DTC P0138 sets within 100 seconds during the rich test when the above condition is met.

Decel Fuel Cutoff Test
* The PCM detects that the HO2S 2 parameter is more than 900 mV.
* DTC P0138 sets within 40 seconds during the decel fuel cutoff test 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 8:

Step 9 - Step 16:

Step 17 - Step 18:

The number below refers to the step number on the diagnostic table.
2. If the voltage does not change more than the specified value, the condition is present.