i had the same thing happen the other day, except mine is a Carrier, I to thought that the line was'nt blocked as I had cut a T in it that I pour bleach in regularly, mine has a secondary drain hole just above the threaded one, long story short I cut my pvc drain to disassemble and when I unscrewed the threaded coupling out I found a big wad of algae had stopped it up partially right there prior to where I cut in the T so no bleach was getting there, I cleaned the coils and bleached out the pan and now it works like a champ, may not be your problem but thought it might be worth a look.