I've got a Miller M1M Furnace. The thermostat kicks on, the combustion fan activates and then the red light flashes twice. The control light status says 'Pressure switch open with inducer on.' I'm guessing this means I need a new pressure switch. Any way to test the pressure switch to make certain? Or should I just replace the switch? Any help's appreciated.

You need to see if it is a normally closed or normally open pressure switch, labeled on the switch, and then use a meter to check for continuity. I would check before operation and after operation. Your inducer motor could be bad too.