I have an american Standard Freedom 90 Gas Furnace that does not light. There is a flashing light on the control board that is flashing 3 times. The thermostat is calling for heat. The code for 3 flashes says pressure switch error. What trips the pressure switch? I found what i believe to be the switch ( micro switch attached to a small diaphragm ) with clear hose attached. What do I look for in troubleshooting the problem. Thanks in advance for your help.