I have a carrier weathermaker 8000.Furnace cycles but will not flame.I replaced glow coil and cleaned flame senser.The pressure switch is in open position.Any ideas?

If pressure switch doesn't pull closed when draft inducer is running, then you have a plugged vent, plugged hose or port where pressure switch hose hooks to, bad pressure switch, faulty draft inducer, or possibly a plugged drain system if the unit is a 90+ unit.