I had the Circuit board on my furnace blinking 3 times. I cleaned out the pressure switch openings and checked the tubes with no success. What I did notice is that when the flue blower came on I heard a swishing sound. I switched off the power to the furnace. Then I disconnected the flue and drain lines and found a blockage in the small black hose which connects to the drain on the side of the furnace (dead insect or debris of some kind). Condensation from the flue was backing up and spilling into the blower. I cleared the drain line and put a small towel at the blower exhaust opening to catch any water. I set the thermostat higher to call for heat and switched on the furnace power. This turned on the blower and with the pressure hoses disconnected, it blew out the moisture into the towel and dried out the blower. After reassembly, everything worked like new. Hope this helps and maybe saves someone a few $$$$.