My issue is with a Carrier Weathermaster 8000 furnace, 58DLA. When I turn on the heat, the air blows but no heat, and after a couple of minutes it shuts off.
A couple of weeks ago the gas line was shut off for a couple of days, and I never tested the heat after that, but the boiler is on the same gas line under the furnace, so I know gas is going through.
The status light blinks constantly, there is no code sequence. This indicates 115VAC polarity reversal, but this is impossible, since no work was done on wiring. Or a twinning issue, but I have only one.
The air conditioning works fine, 120VAC tested fine coming out of the circuit board. I even swapped the 120VAC coming into the furnace, and it still has continuous rapid flashing no matter what.
I've read on similar problems that indicate a grounding issue on different brands of furnaces, but not sure about that out of the blue, it's been working like this for 8 years without issues like this.
Any help would be appreciated.

Check the gas valve and see if there is constan power on it. The coil on the valve could be stuck.

Thanks, it ended up being the board

I traced the board I have to two jfets j211 and j176. I looked at the specs (I'm an EE) and the J211 is an RF jfet. I replaced it with the switching type j112 (and the j176 with a new one). Board works fine. I wonder if they loaded the boards mistaking the j112 for a j211? Both are n channel jfets. Who knows?

Thanks mikebaum, I still have the old board, so I will definitely do what you did to fix it! Won't hurt to have a backup board. Good q's though. Do you have part numbers/descriptions for those 3 jfets? And where is the best place to get them?

My board is in the attic furnace now so by memory: If your board is the same as mine then there is a large chip made by microchip near the middle. Beside the chip is two parts (look up TO-92 case) labeled Q on the board and then a number. On those parts one is a J211 (with other numbers after) in small letters on the flat side of the case. The other is a J176 (with more numbers). I am just guessing here but it seems to me that somewhere there was a mix up. I am hoping this will help others as well. It looks like these parts are part of conditioning the signal for AC sense. In the configuration they are using these JFETs it would be better to use a switch type J112 instead of a radio frequency type J211. The J112 is also a close match for the complement of the J176. The J112 is much more robust than the J211 and that is probably why so many people have had this problem. These parts can be ordered from Digi-Key or Mouser.

Thanks for the details, I'll try that out!