I have a York Diamond 80 Gas furnace approx. 11 years old, and I have a problem w/ Blower not shutting down. Runs constant.. This has happen intermittently, but now it will not shut down at all. Heat works, but once the exchanger cools down the fan still keeps running until the cycle calls for heat again.:confused: What could the problem be?

You need to replace your fan control. Locate the part on wiring diagram on the door of the furnace blower and replace. See sticky notes at the beginning of this board for some pics of fan and limit controls.