My furnace cut out on pressure switch (three blinks on the red LED). The service tech said it was faulty. He missed the real problem which was a blocked condensate trap. Condensate was backing up in the combustion chamber and flooding the pressure switch pickup tube, this resulted in a loss of vacuum causing a shutdown. Repeated automatic restarts with flame ignition and immediate flame-out were occuring after turning the furnace power off and then on again. Before you let the service tech whack you for a new pressure switch, tell him to check the plastic drain hoses, the drain trap, and the pipe to your floor drain.