What causes a furnace pressure switch fault?

Most pressure switch faults are caused by a problem in the draft system rather than a failed switch. Common causes include blocked or restricted intake and exhaust piping, drain issues, cracked or loose pressure tubing, water in the pressure tubing, and a weak or failing inducer motor that cannot pull the required draft.

How do you test a furnace pressure switch?

Homeowners can safely check for a clogged furnace filter, look for obvious blockage at the outdoor intake and exhaust termination (if safely accessible), and check for water pooling near a high-efficiency furnace. What homeowners should not do is bypass the switch or attempt to test by blowing into hoses or swapping parts. A proper technician test uses a manometer to verify draft and pressure, and confirms the switch is opening and closing correctly within the control board's ignition sequence.

How much does furnace pressure switch replacement cost?

If the pressure switch itself has failed, the part typically costs $50–$200. Installed total cost is usually $150–$400 including parts and labor. A diagnostic fee of $50–$150 may apply depending on timing and whether the call is after hours. An exact quote requires an on-site evaluation.

Is it safe to bypass a furnace pressure switch?

No. Bypassing the pressure switch is a serious safety risk. The switch is part of the furnace's combustion safety chain, designed to prevent the unit from running when it cannot safely vent combustion gases. Bypassing it removes a critical protection against carbon monoxide and combustion hazards. A licensed HVAC technician should diagnose and resolve any pressure switch fault.

What Is a Pressure Switch on a Furnace?

Q: What is a pressure switch on a furnace?

The pressure switch is a safety device on a gas furnace that confirms combustion gases can be properly drafted out of the home before allowing ignition. When the inducer motor starts, it creates negative pressure (draft). The switch verifies that draft is occurring, and only then does the furnace proceed with the ignition sequence. A pressure switch fault usually means the draft system failed to prove safe conditions — not that the switch itself has failed.

If you’ve ever seen a furnace flash an error code and the words “pressure switch” pop up when you search the code number, here’s the truth.

The pressure switch is usually not the real problem. Most of the time, it’s the furnace’s way of saying, “I can’t prove it’s safe to run.”

At Colonial Plumbing & Heating, this is an issue we get calls about all of the time. It’s an urgent matter, especially during the winter months or during colds streaks or Northeasters.

Nobody wants to be without heat, so our team got together to answer as many questions as possible regarding this topic. Here’s what our Colonial Techs have to say about it.

What is a pressure switch on a furnace?

On a gas furnace, the pressure switch is a safety device that confirms the furnace can draft combustion gases out of your home before it allows ignition.

Okay, so that is great to know…but what does that actually mean for the average person living in Central Virginia.

To break it down a bit, when the inducer motor starts, it creates negative pressure, also called draft. The switch “proves” that draft is happening, and only then does the furnace move forward with lighting.

So, when you get a pressure-switch-related fault, it often means that the furnace didn’t like what it saw in the draft system. It very rarely means that the actual pressure switch failed.

What is causing the pressure switch fault?

A pressure switch fault is frequently caused by something instead the furnace making it impossible to create stable draft. Common issues we see almost daily while out on service calls include:

Blocked or restricted intake/exhaust piping
Drain Issues
Cracked, loose, or water-filled pressure tubing
A weak or failing inducer that can’t pull the required draft

How to test a furnace pressure switch (what’s safe vs what’s not)

Let’s separate safe homeowner checks from tech-only testing. If there is one thing that our Colonial Techs want to make sure is clear, it is that safety is an absolute MUST. Do not ever try a fix that only a professional is capable of.

What you can safely check:

Make sure the furnace filter isn’t completely clogged
Look outside at the intake/exhaust termination (if you can access it safely) for obvious blockage.
Look for water around a high-efficiency furnace.

What you should not do:

Don’t bypass/ the switch.
Don’t “test” by blowing into hoses or randomly swapping parts.

What a tech does when we test it correctly:
A proper diagnosis verifies draft/pressure with the right instruments (often a manometer) and confirms the switch is opening/closing when it should in the sequence—because the control board is looking for specific behavior, not vibes.

Furnace pressure switch replacement cost

If it truly is a failed switch, typical cost replacement ranges are:

Part only: about $50–$200
Installed total: about $150–$400 (parts + labor), with a diagnostic fee commonly $50–$150 depending on whether it’s after-hours

Remember, these are just estimates. Our Colonial team can give you an exact number once we get to your property and check out the situation.

The Colonial take: Don’t put yourself at risk

If your furnace is showing a pressure switch code, this isn’t a time to jump on YouTube and try some high-level fix. That switch is part of the safety chain that protects your home from venting and combustion problems, so bypassing it is a serious safety risk.

Instead, give us a call. We’ve seen these issues and fixed them hundreds of times throughout the communities in Central Virginia. We’re here to help, so take a moment to schedule your appointment.