The furnace comes on and the furnace ignites. It then burns for a few seconds and goes off without the fan coming on. I have changed the flame sensor. Didn't help. Also the fan will come on by itself without the furnace coming on. Help!!

Check the heat anticipator in the thermostat, set it to .45 and try that, Mike

What is a heat anticipator in the thermostat?

Never mind. I figured it out. It was already set on .45. Still baffled. I checked all the limit switches, The trouble light doesn't flash.

It sounds like the furnace is not recognizing the flame. If the flame sensor or that wire is touching the chassis anywhere it won't work. If the wire to the flame sensor is open somewhere or not securely in the molex connectors it won't work and lastly the PC board could be bad.

Oooooo. And 1 more thing all though it is rare, the flame can't lift off the burner, I don't know how else to explain it to you.