The pressure switch is a small, often-overlooked part with an important job: making sure your furnace only ignites when it's safe to do so. Here's what it does and what typically goes wrong.
The pressure switch confirms proper venting before the furnace is allowed to ignite.
What the Pressure Switch Does
Before the burner ignites, the inducer motor draws exhaust gases through the venting system. The pressure switch confirms this airflow is actually happening by sensing the vacuum it creates. If that pressure isn't detected, the furnace won't move forward with ignition — a safeguard against venting exhaust improperly.
Signs the Pressure Switch Is the Problem
A failing pressure switch typically prevents the furnace from igniting at all. You may hear the inducer motor running, followed by a pause and then a shutdown, sometimes accompanied by a blinking error code on the control board.
Blocked or Kinked Tubing
The switch connects to the venting system through a small plastic tube, and this tubing is, by far, the most common point of failure. Debris, insects, or a simple kink in the line can block the pressure signal even when the switch itself is perfectly fine.
A Weak Inducer Motor
If the inducer motor itself is wearing out, it may not create enough vacuum to satisfy the pressure switch, even with clear tubing. This usually shows up gradually, with the furnace becoming less reliable over several weeks before failing outright.
Condensation in the Tubing
High-efficiency furnaces produce condensation, and if that moisture collects in the pressure tubing instead of draining properly, it can block the signal the same way debris would. This is especially common during long cold snaps, when condensation has less chance to evaporate.
A Cracked or Dirty Diaphragm
Inside the switch itself is a small diaphragm that physically responds to pressure changes. Over years of use, it can crack or accumulate dust, making the switch unreliable even when everything else in the system is functioning normally. At this point, a gas furnace technician will typically just replace the switch outright, since it's an inexpensive part.
Pressure Switch Problems at a Glance
Here's how the common causes compare.
| Cause | How Common | DIY Fix? |
|---|---|---|
| Blocked or kinked tubing | Very common | Sometimes |
| Weak inducer motor | Occasional | No |
| Condensation in tubing | Common in winter | Sometimes |
| Cracked or dirty diaphragm | Occasional | No |
Furnova Furnace Repair, your local expert service provider, checks the pressure switch and its tubing on every no-heat call across North York and the surrounding area, since it's one of the fastest fixes once properly diagnosed.
Because the pressure switch is a safety component, it's not something to bypass or disable, even temporarily. If your furnace keeps failing at this stage, the underlying cause is almost always simple to find and inexpensive to fix.