I have a Comfortmaker Enviro Plus 90 that is not making me comfortable!! I do know that it is an older unit (not sure how old though) as this is an older house. Anyway, when the furnace tries to turn on the blower comes on but it will not ignite. Light below blinks 3 times. Panel says pressure switch is on. What does that mean and is there anything I can do as a quick, temperary fix?