Blower Runs Constantly after Switching from Manual to Auto Mode

[everyman]

After switching from manual mode to auto mode, blower on the gas furnace runs constantly. It will cycle normally in auto mode after shutting the heating/ac unit down and restarting in auto mode. Any ideas on what could be causing this?

The thermostat (Honeywell Chronotherm III T8600C) has also been making clicking sounds that lasts about a minute or so, at irregular intervals several times per week. Everytime this happens, display shows "-AC" on every click. But the clicking has been going on about a year, much longer than the constantly running blower problem.

[hvacwiz]

when your t-stat shows -ac- that means you have lost 24v power to your T-stat, usally caused by a limit switch that is open, when this happens your fan will run all the time. what type of furnace do you have?

[everyman]

The furnace is Lennox Gas WhisperHeat G26 Series. I just had the AC circuit board and capacitor replaced a few months ago.