Oxygen Sensors(O2) must have a minimum temperature of 750° F in order to operate efficiently and produce a precise voltage signal. The shorter the time the oxygen sensor reaches this temperature, the faster the signal will be sent to the ECM (Engine Control Module).
In order for the sensors to reach the temperature they need, they have a heating element inside, which is controlled by the Engine Control Module (ECM) based on the engine coolant temperature and engine load signals. It also controls the circuit of this heating element and allows the flow of current to ground. It also handles the voltage signal received by the heating element circuit and establishes the status of the circuit through comparisons between the detected voltage and factory specifications.