The HO2S requires a minimum temperature to provide a closed loop fuel control system. So the HO2S contains a heater element to reduce its warm-up time and ensure its performance during all driving conditions. The HO2S heater is controlled ON after engine start except for Cold condition and high speed acceleration. The ECM/PCM controls this heater element by duty cycle. The main relay supplies voltage to the heater and the ECM/PCM provides a ground circuit for activating the heater.