Got home and found house cold, thermostat calling for heat. Went to circuit board and found "OK" light blinking 4 times, which translates to closed pressure switch. Tested switch with wires connected and it showed open, hmmm. So I disconnected wires and it still showed open. Disconnected rubber tube and drew slight vacuum by sucking on switch and confirmed contacts close. So it seems like at least now switch is working properly. Cycled the power on furnace to clear lockout and it came on and functioned normally.
Am I correct in diagnosing this as a bad pressure switch? I'd feel better if I could find switch in the failed condition. Is there any other possibility I'm not thinking of?