What makes a furnace venter pressure switch close?
My furnace will not come on. The LED flashes 3 times, which tells me the venter is plugged or my pressure stch is open. I bypassed the pressure switch and everything works fine. I installed a new pressure switch and the furnace came on as normal and the LED stayed on as it is meant to. I put the covers bck on the furnace. Te next evening I turn the furnace on and the same thing happened... LED blinks 3 times... pressure switch not closing... I can bypass it again and fuace works. Any suggestion?