I have an xe80 gas furnace heater turns on but no heat and fan keeps running cold air. What's wrong with my furnace?

These furnaces are very simple appliances. There is a circuit to prove the pilot light and it depends on a clean flame rod. It looks like a platinum spark plug on one end that has the thin platinum rod extending into flame. When it gets dirty it will not sense any flame and will shut off gas valve. Turn off power and pull out burner assembly and clean it all out. All you do is to disconnect the things attached to the burner and lift it out where you can see what you are doing. Get it all clean and vacuum out the heat exchanger and all around. After reassembling check it out and see what is happening. You are going to have to use a meter to get anywhere because in a few minutes you should find the problem. Check your fan relay and t-stat and see if the relay is stuck or it's sending power from t-stat to fan relay coil (24v). Here are some booklets to help: https://dl.dropbox.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf
https://dl.dropbox.com/u/8010163/AC%20Wiring%20Diagrams.zip
https://dl.dropbox.com/u/8010163/Make-up%20Air%20for%20Gas%20Appliances.zip