If your furnace is short-cycling — starting up, running briefly, then shutting off — a failed pressure switch is one of the most common culprits. It's also one of the most frequently misdiagnosed. Here's a clear explanation of what the pressure switch does, why it fails, and how to test it before you spend money on an unnecessary part.
What Is the Furnace Pressure Switch?
The pressure switch is a safety device that verifies the inducer motor (draft inducer fan) is running and creating proper negative pressure in the vent system before allowing the gas valve to open. It's a diaphragm switch that opens and closes based on air pressure — specifically, the negative pressure (suction) created by the inducer motor.
In operation:
- Thermostat calls for heat
- Inducer motor starts, creating negative pressure in the heat exchanger / vent system
- Negative pressure pulls the pressure switch diaphragm, closing the switch contacts
- Control board sees closed switch = inducer confirmed running = safe to proceed
- Hot surface ignitor or spark ignitor energizes
- Gas valve opens, burners light
If the pressure switch doesn't close, the control board interprets this as a venting problem and won't open the gas valve. This is a deliberate safety interlock — if the vent is blocked, combustion gases (including CO) could back up into the home.
How Many Pressure Switches Does a Furnace Have?
Most single-stage furnaces have one pressure switch. Two-stage furnaces typically have two — one for low fire, one for high fire. Modulating furnaces may have more. High-efficiency furnaces often also have a secondary pressure switch monitoring the inducer outlet.
Common Causes of Pressure Switch Failure
1. Blocked or Restricted Vent
The most common cause is not the switch itself — it's a vent restriction. Bird nests, wasp nests, ice blockage (common in Minnesota winters on exterior PVC terminations), or kinks in flexible duct can restrict the vent enough that the inducer can't create sufficient pressure to close the switch.
2. Clogged Condensate System (High-Efficiency Furnaces)
If the condensate drain is plugged, water backs up into the vent system and can reach the pressure switch port, filling the hose with water instead of air. The switch can't operate properly. See our condensate system guide.
3. Failed Inducer Motor
If the inducer motor won't start or is running slowly, it can't create enough negative pressure to trip the switch. The switch itself may be fine — the inducer is the problem. See our inducer motor guide.
4. Cracked Pressure Switch Hose
A rubber hose connects the vent manifold to the pressure switch. These hoses crack with age, losing the pressure differential. This is a $5 fix if you find it — check the hose carefully before replacing the switch.
5. Failed Switch Diaphragm
Eventually, the diaphragm inside the switch cracks or hardens and no longer responds properly to pressure changes. This is the actual switch failure — it's the least common cause but gets blamed most often.
6. Incorrect Switch Rating
Each furnace uses a pressure switch rated for a specific pressure (e.g., -0.50" W.C. or -1.20" W.C.). Installing the wrong switch — even from the same brand — causes nuisance trips or failure to close.
How to Test a Pressure Switch
Warning: This involves working around a live furnace. If you're uncomfortable with electrical work, call a technician.
Basic Continuity Test
- Disconnect power to the furnace
- Remove the pressure switch hose from the port on the switch
- Set a multimeter to continuity or resistance
- Connect probes to the switch terminals (with power off)
- Blow gently into the hose port — the switch contacts should close (continuity)
- Release pressure — contacts should open (no continuity)
If the switch doesn't respond to pressure change, it's failed. If it responds correctly, the problem is upstream (vent restriction, inducer, condensate, hose).
Pressure Switch Cost and Replacement
Pressure switches are inexpensive parts — typically $15–$40 for most Goodman models. Labor for a technician to diagnose and replace is $100–$200. The key is making sure the switch is actually the problem before ordering — the diagnosis steps above help confirm this.
Always use the OEM pressure switch rating. Check your furnace model number and cross-reference the exact part number needed.
Diagnostic LED Codes
Your Goodman furnace's LED diagnostic light will flash a specific code pattern when a pressure switch fault occurs. A 2-flash code on most Goodman models indicates "pressure switch stuck open" — meaning the switch didn't close when it should. Learn to read Goodman LED codes here.
When to Call a Pro
If you've checked the vent, condensate, hose, and inducer and still have pressure switch issues — call a licensed HVAC technician. Persistent pressure switch problems can also indicate a failing heat exchanger (cracked exchangers can alter pressure differentials). Don't ignore recurring pressure switch faults.
Related: Furnace Noises Guide | High Limit Switch Tripping | Control Board Failure
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