My Nest gen2 thermostat experiencing problems offline, low voltage etc … After extensive troubleshooting with tech support i was advised by tech support to replace my gen 2 thermostat. After purchasing a new gen 3 I disovered that the battery of my out of warranty gen 2 was not working properly. After installing a “C” wire I deduced that the battery was at fault. After replacing the battery and using the thermostat it works as intended. So when you are chasing a problem with nest thermostat and a resolution seems elusive Try the battery, the solution was much easier than the process of tracking it down. The battery would cheaper than the time I put in with tech support.