With Vehicle Control Module (VCM)

Diagnostic Trouble Code (DTC) P0139 indicates an open in the Oxygen Sensor (O2S) circuit. DTC P0171 indicates a low voltage (lean) in the Oxygen Sensor (O2S) circuit DTC P0172 indicates a high voltage (rich) in the Oxygen Sensor (025) circuit. If a DTC is set, the engine will always run in the "Open Loop" mode. The Oxygen Sensor (02S) voltage output can be measured with a digital voltmeter (J 39200) having at least a 10 megohm input impedance Use of a standard shop type voltmeter will result in an inaccurate reading. Refer to DTC Charts/System Diagnosis under Procedures. Testing and Inspection

Normal scan voltage varies between 100 mV to 999 mV (0.1 and 1.0 volt) while in "Closed Loop." DTC P0139 sets in one minute if voltage remains between 0.35 and 0.55 volt.

Using the scan, observe the long term fuel trim values at different RPM and air flow conditions to determine when DTC P0171 or DTC P0172 may have been set. If the condition for DTC P0171 exists, the long term fuel trim values will be around 150. if the condition for DTC P0172 exists, the long term fuel trim values will be around 115.