I have a Lennox Pulse furnace that has been having some leaking problems - both internally and externally. The symptoms included the furnace shutting off occassionally and needing to be reset - via reset switch, thermostat or other means. It got worse when I came downstairs one day and the basement was soaked with water! The secondary heat exchanger had apparently been leaking water, and it was backing up the drainpipe, causing the furnace to shut itself off.
Your carrier may not be of this type, but if it is, it could be a very expensive problem. My home warranty company (I haven't lived here too long) is currently negotiating replacing the furnace vs. replacing just the secondary heat exchanger. They've already replaced the muffler and a bunch of the PVC on the exhaust and drainage side.