First it is a Carrier model 58pav heat only no AC, it is about 10 years old and never given us problems. I am not an electrical type person so speak so I understand what you’re saying please. The unit when you turn up thermostat for heat it clicks at the electrical board and then blower starts up the red code light gives no code but flashes at a constant rate. The unit does not stop running the blower unless you turn down the thermostat or cut the power. I called a repair man he said the board is shot and the unit is showing that it has a reverse polarity by the flashing light. I can’t get a person in for over a week and the board is about $325 and that’s allot of money if it is not it. Does anyone know if that is what it is and where to order online parts?

Did you try reversing the polarity? (unplug the furnace and reverse the two wires connecting the plug in the wall to the furnace, black and white) The furnace will not work in heat if reversed. Good luck, Mike

I was going to do that, but my thought was why would it have worked all these years and now it changed?

Well it has been getting colder so purchased the new board, and installed it. I have heat now.