The car has an A/F up front, not an O2. The P0031 means the A/F heater circuit is bad or open, but it's on a different fuse from the ECM relay and I doubt it's related. The rear O2 heater is on the same circuit as the A/F heater, so I don't think it's even a power issue. It's possible (albeit unlikely) that the ground-side heater driver in the ECM blew and took out the ECM's main supply, but I'd check the coils before looking for anything exotic. You may have one that's about to die and heating up intermittently. Let the car get cold and then start it, then feel the coils as the car runs. If any of them heat up very quickly compared to others, that's probably your culprit. Check your engine ground with a voltage-drop test, too. Versas are notorious for dicey ground connections at the bottom end of the cable where it attaches to the tranny, and crappy grounds will definitely cause coils to melt.