A pressure switch (sometimes called a draft pressure switch) is a small diaphragm safety switch on modern furnaces. It senses the air pressure created by the inducer motor and confirms proper draft is established before the control board opens the gas valve.
If the pressure switch doesn’t close — because the inducer isn’t running, the vent is blocked, or condensate is restricting the inducer — the gas valve never opens and the furnace fails on a lockout code.
Common pressure switch issues
- Blocked vent — ice, bird nests, or debris in the flue pipe
- Condensate buildup in the pressure switch tubing (high-efficiency units)
- Cracked switch diaphragm — internal failure
- Inducer motor weak or failed — insufficient pressure to close switch
- Disconnected sensor hose
Pressure switch issues are a frequent cause of “furnace won’t start” calls in Colorado. Replacement is typically $185-$325 including parts and labor.
Related: Inducer Motor, Furnace, Combustion Air.
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Right Way Mechanical is a family-owned HVAC contractor serving Wheat Ridge, Arvada, west Denver, and the Colorado foothills.