I have an old TX500. Even though the fan setting is set to "Auto" (there is no "Off" setting,) the fan keeps cycling on for about 5 minutes, then off, then on etc. It's been doing this for 10 hours so far. I tried resetting the whole automatic program, no luck. Heat system is set on "Off." No AC. This happened before. It stopped on its own after about 5 days, but it was hard on my electric bill. Heater guy said it wasn't the heater.