Heated Oxygen Sensor

CAUTION: Measure Oxygen Sensor (O2S) voltage with a digital volt-ohmmeter (minimum 10-megohm impedance) only. Current drain of a conventional voltmeter could damage sensor.

The Heated Oxygen Sensor (HO2S) is mounted in exhaust system and monitors oxygen content of exhaust gases. See Figure . The HO2S are used for fuel control and post catalyst monitoring. Each HO2S compares oxygen content of surrounding air with oxygen content of exhaust stream. When vehicle is first started, the PCM operates in an open loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference or bias voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage once in closed loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature and to provide an accurate voltage signal. If PCM detects HO2S voltage is not within a calibrated range, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 3.4L MONTANA, SILHOUETTE & VENTURE article.