I have a 1993 Day & Night air and heating combo, when in the heating position the gas burners will not come on. When I disconnect the power disconnect on the side of the unit and plug it back in the heat will start. It will function properly for about two to three weeks without any problems. The heat then quits functioning and I have remove the power disconnect fuse block again to get it going,

When the furnace stops doing anything but should be on, look for a flashing diagnostic light on the PC board. Then, on the blower door, the flashing codes will be explained.