What are the symptons, does the thermostat make contact, does fan come on, does the sail switch come on, does the ignitor come on, does the pilot light come on, does the valve open, does the flame come on, is the gas turend on? You need a little information before you start throwing money at things to see what works. Read up on how the furnace works on the net then you can figure out where the break in the chain of actions is. My guess is you just need to clean the "flame proofer" (90% of the time that fixes furnace problems), but that is only a guess without any symptons.