My thermostat was acting up (old mechanical mercury unit). And I decided to replace the unit. During the installation I noticed that the power to the thermostat went away (verified by a meter). Note that the power existed prior to me attempting to replace thermostat. And now no power after my R&R. Yes, Guilty as charged. So I need to regroup.
I replaced the fuses on the compressor.
I've tried hardwiring AC (RC Y and G)
I reset the AC and FAU breaker, and lastly all the breakers on the panel.
Still no power to the Thermostat.
The Honeywell ST9120C4016 Fan Control board on the FAU is an older board - thus no fuse on it.
My house is dual zone. And the other zone is working fine.
So an option for me would be to swap the Fan Control board from the working unit to the other.
But given my earlier luck, I'm cautious.
Any input on how to figure out why no power to to the thermostat?