P0139

Schematic:

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV under normal operating conditions. The Engine Control Module (ECM) produces a bias voltage on the HO2S signal circuit of 420 to 480 mv. The reference ground for the sensor is provided through the ECM. The ECM monitors the signal voltage to determine if the exhaust is lean or rich. The oxygen sensor voltage is high when the exhaust is rich, and low when the exhaust is lean. The ECM constantly monitors the HO2S signal during the Closed Loop operation.

CONDITIONS FOR RUNNING THE DTC
^ The rear HO2S is in Closed Loop.
^ The battery voltage is more than 11 volts.

CONDITIONS FOR SETTING THE DTC
^ The rear HO2S is stuck above 600 mV.
OR
^ The rear HO2S is stuck below 600 mV.
OR
^ The oxygen sensor voltage is more than 200 mv during decel fuel cut off operation.

ACTION TAKEN WHEN THE DTC SETS
^ The ECM will illuminate the Malfunction Indicator Lamp (MIL) on the second consecutive drive trip that the diagnostic runs and fails.
^ The ECM will record operating conditions in Freeze Frame on the first drive trip that the diagnostic runs and fails, fail pending.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The ECM turns OFF the Malfunction Indicator Lamp (MIL) after three consecutive drive trips that the diagnostic runs and passes.
^ A History DTC clears after forty consecutive warm-up cycles if no failures report by this diagnostic or any other emission related diagnostic.
^ The scan tool clears the MIL/DTC.

DIAGNOSTIC AIDS
Use the J 35616-A connector test adapter kit for any test that requires probing the ECM harness connector or a component harness connector.

If the conditions to set this DTC are not present, check for the following conditions:
^ Heated oxygen sensor wiring-Check for the following conditions:
- A signal wire intermittently open, shorted to ground, or with excessive circuit resistance
- A reference ground wire intermittently open, shorted to ground, or with excessive circuit resistance
- The sensor pigtail or the harness may be mis-positioned and contacting the exhaust system.
- An oxygen supply inside of the HO2S is necessary for a proper operation. This supply of oxygen is provided through the HO2S wires. All of the HO2S wires and connections should be inspected for breaks or contamination. If any wiring requires repair, refer to Wiring Repairs in Diagrams.
- Inspect the connectors for the following conditions:
^ Proper mating
^ Broken locks
^ Improperly formed or damaged terminals
^ Corrosion and water intrusion

^ A contaminated HO2S-Contamination can include the following:
- Fuel contamination
- Use of incorrect RTV sealant
- Excessive oil or coolant consumption

^ Exhaust leaks-An exhaust leak may cause the outside air to be pulled into the exhaust gas stream past the HO2S, causing this DTC to set. Check for any exhaust leaks near the HO2S.
^ Improperly torqued HO2S
^ Inoperative HO2S heater
^ Poor ECM grounds

TEST DESCRIPTION

Steps 1 - 8:

Steps 9 - 18:

Steps 19 - 22:

The numbers below refer to the step numbers on the diagnostic table.
3. Conditions located in the Freeze Frame may indicate if the test failed during normal operating conditions or during a decel fuel cut off condition. If the test failed during a decel fuel cut off condition, operate the vehicle under a decel condition and monitor the HO2S voltage while watching for high sensor voltage during this operating condition. High HO2S voltage during a decel fuel cut off operation indicates that a rich condition exists while the injectors are turned off. Leaking fuel injectors or a leaking fuel pressure regulator may be the cause.
7. Depending on the operating conditions, the heater operation may be modulated or operated continuously. This will be indicated by the test lamp flashing or staying illuminated. This is considered normal operation.
8. The ECM provides the reference ground for the oxygen sensor. A small amount of current will appear on this circuit when the ECM is operational. The measured resistance of the circuit will increase when the ECM is operational. This is considered normal.
10. The ECM provides the reference ground for the oxygen sensor. A short to ground on this circuit will cause the signal voltage to shift.
17. An oxygen supply inside the HO2S is necessary for proper operation. This supply of oxygen is provided through the HO2S wires. Inspect all HO2S wires and connections for breaks or contamination.