First, the control board of the switch should be checked for a mechanical obstruction or faulty wiring and an ohmmeter used to determine if the pressure switch is working properly. A wire brush can then be used to clean the pressure switch plunger and drain tube.
Look for a mechanical jam preventing the switch from operating or for faulty wiring. In addition, water may build up in the hoses running to the switch and cause a switch malfunction. If nothing is wrong with the control board, the pressure switch should be examined using an ohmmeter. Turn off the power to the furnace, blow or suck into the power hoses, and test the pressure switch with the ohmmeter. Check the exhaust ports of the secondary exchanger and inducer for blockages, and look for garbage in the condensate trap.
If the switch is stuck, the plunger may not be pulling in fully when pressure increases in the furnace. A cloth and wire brush are recommended for cleaning dirt from the inducer and secondary exchanger, while the drain tubing may be cleaned by simply blowing through it. Sometimes the condensation trap needs to be adjusted as well so water reaches the drain and traps furnace gases.