Thermostat won't keep furnace fan running

You probably have a loose or broken wire to the fan relay. Check the wires on the thermostat first. Pull up on the bottom of the thermostat to remove it from the wall plate. You should see several screw terminals and the conductor wires that should be connected to them when you look at the wall plate. Check if any of the wires are disconnected or broken. If a wire is broken, strip a portion of the unbroken section of the wire and secure it to the appropriate screw. (The fan relay terminal is G for most setups.) If none of the wires are loose or broken, then you have to check for damage to the wires between your heating system and the thermostat or at the heating system. It could be a bad wire going to the jumper on control board or damage to the board itself.

I hope this helps. If the problem is not at the thermostat, you may need to have a professional diagnose and fix the problem. However, if you post additional comments about your heating system, Fixya experts may be able to help you diagnose the problem.
Cindy Wells
(we had the broken wires at the thermostat keeping the fan from working in the past. It's an easy fix if you are willing to handle small wires carefully.)