P0138

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematic:

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) is a sensor used to detect oxygen-content in the exhaust. The Powertrain Control Module (PCM) supplies the sensor with a signal circuit and a ground circuit. The PCM supplies a bias voltage between the circuits. The sensor varies the voltage based on the oxygen content in the exhaust. When the exhaust stream is lean (more oxygen), the sensor produces a low voltage signal. When the exhaust stream is rich (less oxygen), the sensor produces a high voltage signal. The PCM monitors the HO2S signal during closed loop. To correct for rich or lean conditions, the PCM adjusts the injector pulse width. If HO2S bank 1 sensor 2 remains too high for a calibrated period of time, DTC P0138 will set.

CONDITIONS FOR RUNNING THE DTC
^ No TP sensor, MAF sensor, MAP sensor, IAT sensor, ECT sensor, EVAP system, AIR system (if equipped), fuel injector, EGR system DTCs are set
^ TP angle is between 3-25 percent
^ System voltage is between 9-18 volts.
^ Closed loop fuel control is enabled.
^ Air fuel ratio is between 14.4-14.9
^ Catalyst test inactive
^ AIR test inactive
^ Conditions above met for 5 seconds

CONDITIONS FOR SETTING THE DTC
HO2S bank 1 sensor 2 signal voltage remains above 950 mV (0.95 V) for approximately 112 seconds.
OR
HO2S bank 1 sensor 2 signal voltage remains above 200 mV (0.2 V) for approximately 11 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic test runs and fails.
^ The PCM stores the conditions present when the DTC set as Freeze Frame and Fail Records data.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn the MIL OFF after the third consecutive trip in which the diagnostic runs and passes.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using the scan tool Clear DTC Information function.

DIAGNOSTIC AIDS
If the condition is intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. Verifies whether or not the HO2S sensor voltage is switching from below 300 mV to above 600 mV or remaining above 950 mV.
4. Verifies the bias voltage from the PCM and the condition of the circuit wiring. With the sensor disconnected, the scan tool should display a bias voltage near 450 mV.