why are you guys referring to it as "heater circuit"
the heater circuit is integrated into the o2 sensor itself, test the wiring to the sensor, 2 wires control the heater, one is 12 V +ve supply to the heater, the other is the ECU control which controls the ground leg (which in turn heats up the sensor element)
most of the time such issue is a damaged wire in the loom, its not very hard to repair.