No heat code 3 and code 6. Cleared code, vacuumed all drains, pressure tubes and gutters. Cleared codes, cycled furnace. Furnace is in an unconditioned room, drain could also be freezing up on cold nights. Adjusted drain line in a few spots so it had a good even pitch, ran furnace for 40 mins had great water flow to outside. Increased furnace intake air volume, everything working great at this time