A clogged condensation drain can trip the furnace pressure switch because it senses the accumulation of fluids in the furnace drain pan. A switch stuck in the open position prevents the furnace from turning on. Water sitting in the furnace drain pan also often contains bacteria that can create slime.
The slime problem can be controlled by using special conditioner pan tablets. These slime-preventing tablets are pushed into the drain pan through the drain hole and should be used once a month. Exchanging the existing drain hose for a larger one also helps prevent clogs in the tubing. The condensation drain also needs periodic cleaning of accumulated dirt and debris.