Heated Oxygen Sensors (HO2S)

The rear Heated Oxygen Sensor (HO2S 2) is used to monitor the oxygen storage capability of the catalytic converter. Similar to the O2S 1, the HO2S 2 reacts with the oxygen content in the exhaust stream to produce a voltage signal. This voltage ranges from approximately 0.1 volt (high oxygen - lean mixture) to 0.9 volt (low oxygen - rich mixture). When the catalytic converter is functioning correctly, the HO2S 2 will be fairly inactive. The PCM compares readings from both the front O2S 1 and the HO2S 2 to determine the catalyst efficiency. The PCM will illuminate the MIL if the catalyst efficiency falls below a calibrated level.