When my furnace is ready to come on the igniter will spark and pilot will come on but then almost immediately the pilot flame begins to go up and down in rapid succession. The pilot light will eventually go out after a few seconds. During this short time the igniter is still clicking and i hear a thumping noise coming from somewhere. Any ideas?
no information from the model # you provided.
But it sounds like the flame sensor is bad, or dirty. Since this is what senses the heat, and keeps the pilot lite on.
Pull the assembly off, or if you have room, take a small piece of sand paper and lightly rough up the surface of the sensor. be easy. sometimes they get a coating on them, and you cant even see it.
Try that and post back