Helping a friend that bought a house with this furnace. It was not running right as the red wire for the t stat was not hooked up. I opened the unit as I was getting 3 red lights for the pressure switch being stuck open. I held it closed with my knife and it fired off, then 2 lights for being stuck closed. I removed the P switch and shook it a bit and put it back on. There is a connection for the clear air hose in the back and in the front. The back hose was installed but the front hose was off. I got the unit to run with this hose off but am not sure if it is suppose to be off or on. When installed the furnace fires off then quits and give me the lights. Off, it runs fine. Any one know if this is to be on or off? If its suppose to be on, the switch is most likely bad.
Thanks in advance,