The White wires are fed from F18 (10amp). At the connector (Pin1) the wire is GN/YE (front) or GN/BU (rear). The other side of the heater (Pin2) the return wire to the PCM/ECU is BK/YE or BK/BU. Ford state the heater resistance is 7-15ohms.
One can never be sure, but I believe its just a power transistor to ground (monitored for DTC) during warm up of the exhaust/sensor. Once it get sees a sensible signal from the sensor (switching ~0.15v or ~0.85v) on the black and grey wires, the heater current is switched off.
In fact if the DTC is cleared at this point, it doesn't come back until the next cold (sensor) start.
I use to look inside the connectors and think they were clean, but a couple of times (and others have found this) a squirt of switch cleaner/lubricant (WD40 if real stuff not around) and the fault clears.
Martin