Honeywell Pro T6 Thermostat Power Source Changed

Installed the Honeywell T6 a few months ago. Originally had battery power source, but later had AC guy add C wire power. I reset thermostat for C wire, but left batteries in. Working fine till two days ago when it stopped responding to Dashboard requests. Device Events shows Advanced Honeywell T6 Thermostat AC mains disconnected and it changed to Battery. The thermostat does not respond. It is at a remote location. Is there anything I can do short of making a 3 hour drive to fix this? Sure would appreciate any help.

While it is possible that there's a fault in the thermostat, the alternative possibility is the air-handler/furnace is no longer providing power to the thermostat. That would provide one reason why it switched back to battery.

Thanks for your reply. If changed to battery shouldn't the thermostat still be able to respond?

My T6 never did work on C wire power alone. Honeywell sent me a replacement and it was the same. It kept losing power and resetting. My solution was to leave the batteries in. It worked fine after that. My only suggestion is maybe the batteries are too low?..

The answer to this problem is related to the sequence of steps during setup. See the Installer Manual:

"The T6 Pro Z-Wave thermostat works in the optional Z-Wave battery mode or normal power mode based on its power source. The Z-Wave power mode can only be changed when thermostat is NOT included in Z-Wave network. You can check the power mode in the thermostat menu under MENU/DEVICE INFO."