P0138

CIRCUIT DESCRIPTION

The PCM provides a 0.45 volt reference signal to the oxygen sensor on CKT 1665. When the oxygen sensor is cold, below 200°C (392°F), the oxygen sensor signal voltage will be around 0.45 volt and the PCM will keep the system in Open Loop operation. When the oxygen sensor is warm, above 200°C (392°F), the oxygen sensor will swing from rich to lean rapidly, if the PCM is in good control of the air fuel mixture. DTC P0138 is designed so that if the oxygen sensor stays at a rich voltage for 40 out of 50 seconds during the test conditions, this DTC will set. DTC P0138 will set when:

^ There is an oxygen sensor circuit fault giving a false rich indication.
^ The air fuel ratio is actually rich due to a fuel control or emission system fault.

CONDITIONS FOR SETTING THE DTC

Test Conditions

^ DTCs P0101, P0102, P0103, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0151, P0152, P0153, P0154, P0300, P1133, P1153 and P1154 not set.
^ Throttle Position (TP) sensor between 5.4 and 25 degrees.
^ Coolant temperature 81.5°C (179°F) or greater.
^ Closed loop fuel control enabled.
^ Oxygen sensors ready.
^ Engine speed 800 RPM or greater.
^ MAP sensor greater than 32 kPa.
^ Conditions above met for 3 seconds.

Failure Condition
Oxygen sensor has been rich (534 mV (0.534 V) or greater) for 40 out of 50 seconds.

ACTION TAKEN WHEN THE DTC SETS

^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
^ A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any emission related diagnostic test.
^ PCM battery voltage is interrupted.
^ Using a Scan tool.

DIAGNOSTIC AIDS

Notice: Never solder heated oxygen sensor wires. Loss of the Air Reference to the HO2S Sensor will occur. For proper wire and connection repair, refer to Diagrams / Diagnostic Aids.

Check snapshot data for front and rear long term fuel trim values. If both banks were rich, check for a condition that would cause rich engine operation.

TEST DESCRIPTION

Number(s) below refer to the step number(s) on the Diagnostic Table.

2. If DTC P0140 is also set, diagnose it first because an Oxygen Sensor not ready can cause other emission related problems.
3. If the Oxygen Sensor is operating correctly, voltage will fluctuate below 0.6 volt. A rich sensor/condition will cause the voltage to remain above 0.6 volt.
4. Checking for proper oxygen sensor operation. The voltage should fluctuate below 0.6 volt.