On Dec 6, 12:49*pm, wrote:
All of a sudden my furnace blower runs constantly even though it is
set on "auto" at the thermostat. The heat cycles on and off properly
(fortunately because it is 19 degrees outside). The furnace is about
10 years old and is gas. I seem to recall there is a wiring option on
the motor for it to run constantly but no one has messed with that
since it was installed. I would appreciate any ideas on what the
problem might be.

First off, disconnect all the wires at the T-stat and see if it shuts
off. If it does, you have a faulty t-stat/wiring.
If it stays on, put the wiring back for the T-stat, and now disconnect
the T-stat wiring at the furnace board to confirm its not the wire
shorted in the walls someplace.
If it still stays on, it could be a stuck relay on the control board
on the furnace. You can try tapping it.
Another possibility is that if the chamber/duct temp sensor could be
faulty, giving a high reading and could be telling the blower to stay
on.