My thermostat failed and I am replacing it with a Honeywell Lyric T6 pro.
The previous configuration had G,W,R connected with Rc jumpered. The thermostat wire was 5-wire, 22ga I think. I used an extra wire to run power from the transformer (28vac) up to the common. However I still cannot get it to turn on. I tried checking voltage at the thermostat but I'm actually not sure how to check without access to the neutral. Using any ground I could find I was getting 6-7 volts. I'm sure this is not valid but regardless, its not turning on.
Also, There is a C terminal on the furnace board. The instructions make it sound like the wire should go straight from the transformer to the thermostat. Should anything be connected to the C terminal on the board?