A pressure switch stuck open means the furnace’s draft inducer ran, but the switch never confirmed adequate airflow, so the board locked out before ignition. Most of the time the root cause is not the switch itself.
What the Pressure Switch Actually Does
The pressure switch is a small diaphragm device mounted near the draft inducer motor. When the inducer spins up and draws combustion gases out of the heat exchanger, it creates a slight negative pressure inside a rubber hose that connects to the switch. That pressure deflects the diaphragm, closes a set of contacts, and tells the control board: airflow confirmed, safe to fire the igniter.
If the switch never closes, the board sees an open circuit and shuts down before ignition. That’s the “stuck open” fault. The furnace is doing exactly what it was designed to do.
The Most Common Causes, in Order of Frequency
Blocked condensate drain (high-efficiency furnaces only)
On 90%-plus AFUE furnaces, the inducer housing and heat exchanger produce liquid condensate that drains through a trap and hose. If that drain is clogged, water backs up into the inducer housing or secondary heat exchanger. Pooled water can block the pressure switch port or the hose connection, throwing off the pressure reading. This is the most common cause on high-efficiency furnaces, especially at the start of heating season when a summer’s worth of buildup has dried in the trap.
Clearing the condensate path properly means disassembling the trap, flushing it out, and confirming flow all the way to the drain. Worth having a tech do this if you’re not sure how your specific unit is configured.
Cracked or disconnected pressure switch hose
The rubber hose connecting the inducer housing to the switch can crack, kink, or pop off the port. When it leaks, the negative pressure bleeds off and the switch never trips.
You can do a quick visual check without tools: follow the hose from the inducer housing to the switch and look for obvious kinks, disconnections, or visible cracking. If it looks intact, that doesn’t fully rule it out since small cracks only open under flex, but obvious damage confirms the problem. Replacing the hose requires matching the inside diameter of the original.
Weak or failed draft inducer motor
If the inducer is spinning but not generating enough suction, the switch won’t close. A bearing starting to fail will often sound sluggish or take longer to come up to speed. Diagnosing this properly requires measuring the actual pressure the inducer generates with a manometer. That’s not a homeowner check.
Faulty pressure switch
The switch itself does fail, but it’s further down the list than most online articles suggest. Rule out the hose and the drain first. A technician can confirm a bad switch in a few minutes with direct pressure testing at the switch port.
Blocked flue or intake pipe
On 90%-plus furnaces with PVC venting, debris, ice, or birds can block either the intake or exhaust termination. Reduced airflow starves the inducer of the pressure differential it needs. Check both pipe ends at the exterior wall. If one is blocked, clearing it may resolve the fault.
What a Tech Actually Checks
A technician works through this in order: inspect the hose, test the switch with a gauge or direct suction, check the condensate trap and drain path, and measure the actual pressure differential the inducer is generating. That last step is the one that matters. If the inducer isn’t reaching the switch’s rated trip pressure, the problem is the motor or the vent, not the switch.
This diagnosis typically takes 20 to 30 minutes. Replacing a pressure switch or hose is a quick repair once you’ve confirmed the root cause. A failing inducer motor is a larger job involving electrical components, and on newer units there may be warranty considerations depending on age and service history.
A cracked heat exchanger is the other concern worth mentioning: if combustion gases are escaping into the airflow side, that’s a CO risk and requires a trained tech with the right test equipment to evaluate. Don’t skip that inspection if short-cycling has been going on for a while.
What You Can Check First
Before calling, go through these:
- Power: Is the furnace getting power? Check the breaker and the service switch on the unit.
- Filter: A heavily clogged filter restricts airflow enough to cause inducer problems. Pull the filter and check it.
- Exterior vent pipes: Walk outside and look at both the intake and exhaust terminations. Birds, ice, or debris blocking the pipe ends will trigger this fault.
If those are clear and the fault persists, that’s where the tools come out.
Call Us
A furnace locking out on a pressure switch fault will keep doing it until the root cause is fixed. If you’re in Tri-Valley or the East Bay, my team at adriumservice.com can run the diagnosis and sort it out, with same or next-day availability on most calls.