To test a pressure switch you must first have a compound pressure gauge (digital or analog).
Tee in a plastic tee for both the positive and negative ports for the pressure gauge, start unit up and observe the reading on the gauge. The reading must be higher than the setting listed on the pressure switch. If the pressure switch only has 1 port then tee in the correct port of the pressure gauge and leave the other port open to atmospheric pressure.
If the reading on the gauge is lower than that of the switch then the problem is in the furnace or venting.
If the reading on the gauge is higher than that of the switch and the switch is not closing, then the problem is the switch.
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