Puzzler on our Furnace
Have you checked the actual 12v voltage at the furnace. Then check your voltage at the battery. Both voltages should be the same. Now start up your furnace and see what the voltage is. It could be the voltage that the board is seeing when the furnace is running is below it’s shut down voltage. I can’t remember if it’s 10.5 or 11v furnace will shut down. You could have a poor ground or your positive cable has a poor connection. Easy to check. Run a wire from your battery positive to to furnace 12v input. Run a ground cable either from battery negative or from a good known ground spot on the frame to the furnace frame.
Run furnace to see if that works. First check your 12v voltage at your furnace. Good luck