A furnace pressure switch is a safety device that verifies the draft inducer is actually moving combustion air before the furnace is allowed to light. The running inducer creates a slight vacuum; a hose carries that vacuum to the switch, which closes and tells the control board it is safe to energize the igniter and gas valve. If proper draft isn't proven, the switch stays open and the furnace locks out — no heat.
Here is the part homeowners most need to know: a tripped pressure switch is frequently a messenger, not the problem. A clogged condensate drain on a high-efficiency furnace, a blocked or cracked vent pipe, a cracked switch hose, a weak draft inducer, or even a very dirty filter can all stop adequate draft and open the switch. The switch is doing exactly its job by refusing to fire. Replacing the switch when the real fault is a blocked drain just relocates the failure.
The switch itself is inexpensive — $50 to $200 for the part, and $150 to $400 installed, around $250, usually under an hour of work. Because the symptom and the true cause diverge so often, the value of a good technician here is diagnosis, not parts: confirming whether the switch failed electrically or is correctly responding to a venting or condensate problem upstream. Ask which one it was before approving a replacement.