I have a Honeywell T6 PRO Programmable Thermostat. It is set on Cool with the Fan set on Auto but the fan Keeps on Running constantly. Can you tell me how I can correct this. Please and thank.

SOURCE: My honeywell Thermostat reaches the heating temp.

I want to give a simple explanation before I go any further. On the heating side of a unit even though tthe temp has satisfied the thermostat, the fan will run until it cools down the plenum where the air flows from the air handler. There is a bimetal switch called a fan limit that operates the blower when the plenum reaches the temp set on the fan limit switch. Now if your fan keeps running even after the plenum chamber has cooled down, then you could have a fan limit switch that is stuck in the run position and must be replaced in order to correct the problem.

SOURCE: I had a Honeywell Thermostat

Is this a new hvac system? A new furnace can work differently than your old one. Does it shut off after a minute or two or continue to run? If it continues to run then call that professional back and let him fix it. If it shuts down after a minute or two then it is a feature of the new thermostat to squeeze all of the cool it can from the a/c coil and ductwork.Some of them have settings that you can adjust for blower delay on and off, read the manual that came with the t-stat and see if it does.