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 to provide an accurate voltage signal. The HO2S 2 is used for post catalyst monitoring. This DTC will set if the HO2S 2 voltage remains above a calibrated amount for an excessive amount of time.

Each HO2S 2 has the following circuits:
^ HO2S 2 high signal
^ HO2S 2 low signal
^ HO2S 2 heater ignition voltage
^ HO2S 2 heater ground

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
^ The system voltage is between 9-18 volts.
^ The Closed Loop commanded air-to-fuel ratio is between 14.5-14.8.
^ The throttle angle is between 3-40 percent.

CONDITIONS FOR SETTING THE DTC
The HO2S 2 signal voltage remains above 975 mV during Closed Loop operation for longer than 45 seconds.

OR

The HO2S 2 signal voltage remains above 200 mV during decel fuel cut-off (DFCO) operation.

Either of the above conditions is present for up to 5 minutes.

ACTION TAKEN WHEN THE DTC SETS
The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the failure records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE DTC
^ The PCM turns OFF the MIL after 3 consecutive ignition cycles that the diagnostic runs and passes.
^ A current, DTC last test fails, clears when the diagnostic runs and passes.
^ A DTC stored in history clears after 40 warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ A scan tool can be used to clear the DTC.

TEST DESCRIPTION

Steps 1-6:

Steps 7-13:

The numbers below refer to the step number on the diagnostic table.
3. The HO2S 2 voltage may default to the Bias voltage. The DTC should be cleared and the Freeze Frame/Failure Records data recorded before proceeding. A normally functioning HO2S 2 voltage signal will fluctuate above and below the bias voltage amount. The action in the test step should result in a visible reaction from the HO2S 2 output.
5. A voltage reading other than bias voltage in this step indicates a short circuit condition in the high signal circuit.
7. The conditions listed in the table may contribute to the failure of the HO2S. The conditions listed apply only to this type of failure.