Rear Heated Oxygen Sensor (HO2S 2)

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, only after the catalytic converter. This voltage ranges from approximately 0.1 volt, high oxygen-lean mixture, to 0.9 volt, low oxygen-rich mixture. Unlike the O2S 1, with a good catalytic converter, the HO2S 2 is normal when the activity appears lazy or inactive. The PCM compares readings from both the front O2S 1 and the HO2S 2 to determine the catalyst efficiency and will illuminate the MIL if this efficiency falls below a calibrated level.