If the air filter is dirty, excess pressure will build up across the filter, triggering the normally closed air pressure switch (PF) to open. 


Possible Cause
Component to Check
Recommended Action
Dirty air filter
Air filter. Open the middle-front panel to access the filters.
Replace air filter.
Switch tubingTubing leading to switchVerify that tubing is clear and that it is not kinked
Incorrect wiring
Alarm wiring
Correct wiring. Refer to the Dirty Air Filter Alarm Wiring diagram below.
Incorrect air pressure switch setting
Arrow on the dial should be pointing at 250
Correct setting to 250.
Faulty air pressure switch
Air pressure switch
If the switch is open while the filter is clean and setting is correct, replace the switch

Dirty Air Filter Alarm Wiring