How do you test a pressure switch and other limit controls? This is for people who are experienced with electrical equipment and the use of a Volt Ohm meter. *Please never by pass a pressure switch or limit control. The pressure switch and limit switches are there for your safety. If you have an open limit control either the furnace has over heated or the limit has gone bad. Problems that would make a limit open up would be dirty filters, dirty evaporator coil causing a restriction in the air flow or a slow blower motor (check the capacitor on the blower motor to make sure it is good). A weak blower motor capacitor will cause the blower to run slow and eventually fail. Pressure switches, and limit switches can be tested with a meter in the same manner. With the furnace calling for heat you can test each of the terminals on the pressure switch to ground to make sure the pressure switch is closed. You should be getting between 24 to 28 volts from each terminal to ground if the pressure switch is closed. If the pressure switch is open you either have a stopped up vent, drain line (if you have a condensing furnace) or bad pressure switch. Please keep in mind there are many different types. The front terminal is burnt and discolored. This is a clue that this pressure switch has a problem.
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