Dead furnace and thermostat feed
Late last year, I swapped out a mechanical Coleman Mach thermostat for a Honeywell digital. The A/C worked fine on the Honeywell but the furnace would not kick on and I ignored the issue believing it...
Forum Discussion
I'd suggest to interrupt the expected power feed from the furnace to the thermostat to confirm that it is, indeed, the power source for the thermostat. That ought to save any unnecessary wallboard removal. I think you'll find that the 12V to the thermostat is parallel to the control board wiring for the furnace.
