Powertrain Chart 8, Front Oxygen Sensor Diagnosis

PCM provides .45-volt reference signal to oxygen sensor on circuit No. 412. When warm, a properly operating oxygen sensor will drive .45-volt reference signal lower, indicating a lean mixture or higher, indicating a rich mixture. This signal will toggle from rich to lean rapidly, at least one toggle every 2 seconds, if PCM is in good control of air/fuel mixture.

This chart is designed to be used after oxygen sensor circuit and PCM operation have been verified through POWERTRAIN CHART 1, POWERTRAIN SYSTEM CHECK and PCM diagnostic trouble code diagnosis.

This chart should identify oxygen sensors that are open (not able to drive .45-volt reference signal) or shorted (sensor output is fixed high or low, due to contaminants or internal sensor faults).

1. With engine running and coolant at normal operating temperature (85°C or greater) and at fast idle, observe engine data parameter PD32. Voltage should swing from less than .3 volt to greater than .6 volt.
2. With engine at fast idle, observe open/closed loop status light (AUTO indicator on CCC). If PCM switches system into closed loop operation, oxygen sensor is okay. If PCM remains in open loop operation and coolant temperature PD04 is greater than 185°F (85°C), replace oxygen sensor.
3. If voltage remains .3-.6 volt for a minimum of one minute, replace oxygen sensor and check for cause of sensor contamination.
4. If there is no voltage variation, check sensor for intermittent terminal contact or faulty sensor.

Fig 1: Powertrain Chart 8 Schematic, Front Oxygen Sensor Diagnosis

Courtesy of GENERAL MOTORS CORP.

Fig 2: Powertrain Chart 8 Diag. Flow Chart, Front O2 Sensor Diagnosis

Courtesy of GENERAL MOTORS CORP.