> For the three wire unit, is the O2 sensor element and the heater element > both grounded to the shell? Is one of the heater wires (two white wires > for the sensors I've seen) just a no-connect inside the O2 case? OOps... My bad. The 4 wire units are the bad ones... on the 3 wire, the O2 sensor return is the case... the heaters are isolated on both.