What brand and model of thermostat is it ?
I found the info,
"E2" on temperature display Brownout condition or too low of voltage to thermostat. Double check wiring and check for24 vac between R and C. E2 will clear 15 sec after proper voltage is restored.
I understand that you are getting 28 volts,,
Is this a heat pump air handler or a gas furnace ?
Is the transformer a 220 to 24 or a 120 to 24 ?
I really don't know what the problem may be , the E2 error means that there is low voltage present at the stat, apparently the stat believes there is low voltage there and will not reset itself. You may end up replacing the stat., I will leave this question open for some of the other experts to look at also.
I replaced the thermostat with another from the my other unit that is working and it gave me the same E2 message. So I am guessing it is not the stat. Maybe the 24V relay outside could be shorted and dropping the voltage to the transformer??
If you have 24 volts at the stat it should work, there are a couple of good Carrier guys here but they probably won't see this until morning.
Just helping Mark..
I had a customer do this same thing and his 24 + and 24- were reversed. He put the transformer in and it simply fed the voltage the wrong way and the stat did not recognize.
May be your problem....maybe not...but worth checking.
Unhook at the furnace...W, Y, G.
leave only the r & c hooked up.
See what that does,
then add w
then g then y
y=condensing unit outside
see if it is one of thos efunctions in the common side
I would check the 2 wire for any nicks ,rubs or contact with metal.
you can discon the coil wires from the contactor..turn off discon...pull the 2 24v wires from the contactor, and see if message leaves.
obviously then if the short goes away with the contactor out of the loop, replace contactor...if still there, it is in the 2 wire.
But if it went away with power off.....Bad contactor
That is correct. What happens,
24 v leaves transformer
24v arrives at thermostat
thermostat decides what function
thermostat sends 24 v to air handle "cool"
air handler "cool"sends the 24v to outside
outside condenser receives 24 at the contactor
24v now leaves contactor to head "home" to common
24 arrives at C for common at the pcb
the pcb sends 24 v common on transformer.
So that is the journey the 24v took until it arrived at C. We want to interrupt the 24v just before it reaches C on the pcb and see if there is no short. If fuse is ok with C unhooked, we know we do not have a short to ground. If it blows fuse hooked to C, then pcb bad
Sorry to ramble so much but I try to simplify the circuit.
I thank you Eric and I am available for follow up on this project at no charge for you.