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What is make and model of the furnace and is the blinking steadt or in a pattern?
Before I give the answer of 99.9% of this code I must ask...has wiring been done recently in home where wires could have been swapped?
I was waiting for a response to my question...has there been any wiring done in the house recently that would have swapped wires?
Then you have a defective circuit board. Whether power outage, or just plain failure, the board no longer reads power correctly.
The only fix is to replace the circuit board.
You are very welcome....and I am sorry that we cannot actually "repair" the furnace online for you.
All I can do is answer your questions as correctly as possible based on my 25 years of working on Carrier furnaces.
Your only fix is to replace the board so at least you know what will be happening.
I wish you the best
If you would like to replace the part yourself I can direct as far as where to get board and how to do it.
Or you call for service.