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 of the exhaust stream. When the vehicle is first started, the powertrain control module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and provide an accurate voltage signal. If the PCM detects that the HO2S 2 voltage remains above a calibrated amount for an excessive amount of time, DTC P0138 will set.

CONDITIONS FOR RUNNING THE DTC
- DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0208, P0410, P0412, P0418, P0419, P0440, P0442, P0443, P0446, P0449, P1415, P1416, P1441 are not set.
- The system voltage is between 9-18 volts.
- The TP Sensor parameter is between 3-25 percent.
- The Loop Status parameter is closed.
- The Air Fuel Ratio parameter is between 14.5 to 14.8.
- The above conditions are met for 3 seconds.

CONDITIONS FOR SETTING THE DTC
The PCM detects that the HO2S 2 voltage is more than 950 mV, for more than 50 seconds.

Or

The PCM detects that the HO2S 2 voltage is more than 200 mV, during decel fuel cut-off (DFCO), for more than 12 seconds.

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

Steps 1-2:

Steps 3-5:

Steps 6-7:

Step 8:

Steps 9-14:

The number below refers to the step number on the diagnostic table.
2. A normally functioning post catalyst HO2S voltage signal will display very little activity. The HO2S 2 voltage parameter should react immediately to the changes in throttle position when performing this test.