P0139

DTC P0139 Diagnostic Chart:

Heated Oxygen Sensor (HO2S) Circuit:

Circuit Description
The VCM applies a bias voltage of approximately 450 millivolts (350-550 mV is normal bias voltage) between terminals "BL23" and "BL24". The heated oxygen sensor varies the voltage within a range of about 1000 mV if the exhaust is rich, to about 100 mV if the exhaust is lean. DTC P0139 is set when the voltage does not vary on CKT 412 within a predetermined amount of time.

The sensor is like an open circuit and produces no voltage when it is below 315°C (600°F). An open sensor circuit or a cold sensor causes "Open Loop" operation.

DTC P0139 Will Set When
^ Engine has been running longer than 2 minutes.
^ Engine coolant temperature greater than 70°C (158°F).
^ Throttle position sensor signal greater than 5%.
^ Oxygen signal voltage steady between 350 and 550 mV.

Action Taken (VCM will default to)
If the conditions for a DTC P0139 exist, the system will not go into "Closed Loop."

DTC P0139 Will Clear When
Conditions for fault are no longer present and Tech 1 clear DTCs function is used or 50 ignition switch cycles have passed with no further faults or ignition switch is turned "OFF" and Fuse #9 (ECM B) fuse is removed for 10 seconds.

DTC Chart Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This test checks the heated oxygen sensor's heating element. The heating element resistance should be 3.5 ohms at 20°C (68CF) or 14 ohms at 350°C (662°F).

2. This will determine if the sensor is at fault.

3. For this test use only a high impedance Digital Volt Ohmmeter (DVM) J39200. This test checks the continuity of CKT 412 and CKT 413. If CKT 413 is open, the VCM voltage on CKT 412 will be over .6 volt (699 mV).

4. If the fuse was open, check the VCM wiring diagram for complete wiring circuit.

Diagnostic Aids
If the oxygen sensor heater is not operating properly, system may go into "Open Loop" after extended idle.

Normal Tech 1 scan tool voltage varies between 100 mV to 1000 mV (.1 and 1.0 volt), while in "Closed Loop." DTC P0139 sets in one minute if voltage remains between .35 and .55 volt; however, the system will go "Open Loop" in about 15 seconds. Refer to "Intermittents". - Intermittent Malfunctions

An oxygen sample inside the HO2S is necessary for proper HO2S operation. This supply of oxygen is directed through the HO2S wires. All HO2S wires and connections should be inspected for breaks or contamination which could prevent reference oxygen from reaching the HO2S.