The wiring color of the oxygen sensor depends upon the make and model. Make and model will dictate which wires are used for what purpose. Usually, the white wires on an oxygen sensor are for the heater. The heater is not powered on all the time, but only when the temperature is too low for the sensor to function properly. The reason for this is that the emissions sensor measures the amount of oxygen in the emission gas. If there is too little oxygen, it means that the fuel mixture is too rich and needs to be adjusted.