An HVAC pressure switch is a protective device that is installed in both heating and cooling systems. The switch will open and cause the system to stop operating under certain conditions which may hurt the system. Pressure switches are commonly tripped by occurrences like clogged vents or hoses, dirty filters or condenser coils. If you are troubleshooting your system, you may also want to test the pressure switch itself.
Inspect the terminals on the pressure switch. If any of them are burnt or badly discolored, then you should replace the switch.
Set the voltmeter or multimeter to display AC volts.
Touch one of the probes from the meter to either one of the terminals on the pressure switch. Ground the other probe from the meter by touching it to any nearby bare-metal surface on the system.
Check the voltage reading on the meter. If you are getting a reading of less than 24 volts, then the pressure switch should be open. If all hoses, vents and coils are free of clogs and clean, you may have a faulty pressure switch.