I have a Honeywell T8011R thermostat. It is blank. I have 120 vac coming in, the transformer has 28 vac coming out. The 5 amp fuse is fine and the control switch is on. On the circuit board with the 5 amp fuse, the brown wire from the transformer shows 28 vac, the double brown wire under it is 0 vac. The red wire under that shows 28 vac. The grey wire under that one shows 0 vac. And the last red wire under that one reads 28 vac. I think the circuit board may be the problem but I am not sure. Is that easy to replace or should I check something else first?

Need a little more info... what type of furnace/air handler do you have (brand/model)? Check for 24 volts where thermostat wires connect to indoor unit (R-C). You may need to hold or tape the blower door interlock switch (if present).

The problem turned out to be a float switch. The air handler is upstairs and has 2 drain lines. The lower one runs outside. The other is capped with a float switch. I was able to measure the volatge of the red wire at the base of the thermostat and it was reading under 3 vac. I next read the red wire in the bundle of thermostat wiring and it had 14 vac. I traced the white wire that was attached to the red wire to the float switch in the pvc drain. When I pulled it out I found it was filled with water. I removed the water in that line and cleared the rust that had built up in the air handler so that it would drain through the primary pvc drain line. It works fine now.