The only circuitry for the furnace that I know of is the main control board. Open the outside cover and its sitting right there on the left. It contains the igniter, gas valve, motor control and timing functions. PPL has them and they run about $100. You will also get an updated version which is coated for better weather protection. I swapped the boards from the front to rear furnace to verify it was the problem. The front one had already been replaced with the updated version. It also a favorite place for mud dobbers. They built a nest over my flame detector and igniter. I believe the high voltage smacked the flame detector. It's a pretty easy job to get at it and swap it out.
Verify the part number before ordering but I believe this is the one I used.