It should have two thermostats, one for LP and one for electric, with a "LP/Elec" knob. The knob turns a gas valve on and off, and moves the toggle switch. I'd check the switch, they do sometimes go bad, and make sure none of the wires have been chewed or are frayed. There really should be no circuit board, other than possibly the electronic igniter, which is independent ot the other wiring.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.