HVAC SENIOR

If your stuck-open furnace pressure switch is more than a technical glitch — it’s a worked-over safety switch doing its job. But when it refuses to shut, your furnace refuses to ignite, leaving your house chilly and inconvenient. Understanding why and how to fix it can save you from having to make unnecessary service calls and keep your furnace running inefficiently and in a way that poses safety risks.

This detailed guide will cover:

•          What is a furnace pressure switch and what does it do

•          Stuck-open switch symptoms

•          Usual causes of causing problems

•          Step-by-step repair and troubleshooting

•.          Prevention

Let’s get started.

What Is a Furnace Pressure Switch?

A furnace pressure switch is a safety device that checks to make sure the inducer fan (also called the draft motor) is working properly before lighting the furnace. Its main job is to make sure combustion gases can safely vent outside your home.

Here’s how it works:

1. Your thermostat calls for heat before the inducer motor starts.

2. The inducer creates a small amount of negative air pressure (vacuum) in the furnace.

3. The pressure switch feels this vacuum and closes to complete the electric circuit.

4. It only lights the burners after that.

If the switch is open, the control board interprets it as dangerous — shutting off the ignition process immediately.

⚠️ What Does “Pressure Switch Stuck Open” Mean?

If your control board displays an error like “Pressure Switch Stuck Open”, it means that the switch did not close even though the inducer fan was running. It interrupts the startup cycle and your furnace won’t ignite.

The reason could be:

•Blocked exhaust or intake vents

•Bad inducer motor

•Kinked or disconnected pressure tubing

•Defective pressure switch itself

In a nutshell — something is blocking proper airflow or vacuum pressure.

Stuck Open Pressure Switch Symptoms

Double check the symptoms before you get out your tools. What you should typically see:

• Furnace won’t ignite or blows cold air
• Inducer fan stays on continuously without ignition
• Flashing error codes on your control board (e.g., 2 or 3 flashes depending on the furnace make)
• Clicking sounds as the furnace continuously tries to fire
• Short cycling — cycling on and off in a few seconds

If your furnace exhibits these symptoms, the pressure switch is stuck open or not properly reading vacuum.

Typical Causes a Furnace Pressure Switch May Be Stuck Open

These are the most typical causes of this problem:

1. Vent Pipe Is Clogged

Dust, leaves, birds’ nests, or ice may clog the exhaust or intake vent and obstruct airflow. Without proper airflow, the pressure switch will not close.

Fix

Inspect and clean any blockages from the intake and exhaust pipes. Make sure they are clear of debris and sloped properly to prevent water from pooling.

2. Condensate Drain Clogging

Condensate generated by high-efficiency furnaces drains via a trap or hose. When clogged, this drain causes water to flow back into pressure tubing or inducer and interfere with airflow.

Fixing:

Inspect the condensate trap and tube. Remove any blockages and check that water drains freely.

3. Cracked or Broken Pressure Tubing

The pressure switch to inducer housing rubber tubing could crack, split apart, or become water-filled.

Repair:

Inspect the tube for cracks or water. Drain out moisture and reconnect it tightly. Replace the tube if it is cracked or brittle.

Also read: Why is my Electric Furnace Blowing Cold Air

4. Weakened or Failing Inducer Motor

If the inducer fan motor cannot rotate fast enough to allow sufficient vacuum to be built up, the switch will not close.

Repair:

Look for unusual sounds or weak airflow at exhaust vent. In case the inducer is sluggish, the inducer may need cleaning, lubrication, or replacement.

5. Faulty Pressure Switch

Every now and then, the switch goes mechanically or electrically wrong — either it doesn’t respond to pressure changes or the internal diaphragm is broken.

Repair:

Test the switch with a multimeter for continuity when the inducer is running. If it doesn’t close even with proper suction, replace the switch.

6. Cracked Heat Exchanger

A cracked heat exchanger can alter airflow and pressure, preventing the switch from operating correctly. Though rare, it’s a serious safety issue.

Fix:

Call a certified HVAC technician immediately. Operating a furnace with a cracked heat exchanger can release dangerous carbon monoxide.

7. Malfunctioning Control Board

At times, the control board of the furnace malfunctions by misreading pressure switch signals.

Repair:

In case all else checks out, check voltage at the control board terminals. If the reading is incorrect, the board might require professional replacement.

Troubleshooting a Furnace Pressure Switch Stuck Open

⚠️ Safety first: Always turn off power to the furnace before making repairs. If in doubt, call a pro.

Here’s a step-by-step troubleshooting process:

Step 1: Power Cycle the Furnace

•Switch off the furnace switch for 1–2 minutes.

•Switch it back on to check if the switch clears.

Sometimes, it’s a fleeting glitch.

Step 2: Check Vent Pipes

•Look for obstructions, snow, leaves, or debris in intake/exhaust pipes.

•Check that pipes aren’t crushed or kinked.

If airflow gets better, your problem may be fixed.

Step 3: Check the Inducer Fan

•Listen for smooth running.

• If it hums but will not spin, the motor could be seized.

• Clear out dust and dirt from fan blades.

Step 4: Inspect the Pressure Tubing

• Pull out the little rubber hose from the switch.

• Gently blow through it to clear blockages.

• Secure it back and check that it is not cracked.

Step 5: Inspect the Pressure Switch

• Verify continuity with a multimeter when the inducer is operating.

• If the circuit fails to close, the switch could be defective.

•If it’s clogged, replace it if needed (they’re not that expensive).

Step 6: Check the Condensate Line for Water

•Use a small brush or compressed air to clear clogs.

•Make sure that the line is sloped downward for proper drainage.

Step 7: Call a Professional

If none of these steps fix the problem, call in an HVAC professional. They have the ability to check pressure levels and identify deeper electrical or airflow issues safely.

Pro Tip: How to Bypass a Pressure Switch (Only for Testing!)

Sometimes technicians will briefly bypass the switch to confirm it’s the issue. But this is only for testing purposes — never operate your furnace without it in place. The pressure switch is a safety device that prevents hazardous gas from building up.

Also read: Furnace Ignitor Lights But No Flame

How to Prevent the Pressure Switch from Sticking Open

Prevention is better than a cure. Here’s how to prevent it from sticking open in the future:

1. Clean Vent Pipes Every Year – Clear away dirt, nests, and blockages prior to winter.

2. Inspect the Inducer Assembly – Keep it free of debris and in good condition for corrosion checking.

3. Replace Air Filters Frequent – Restricted airflow can lead to pressure imbalances.

4. Flush the Condensate Drain – Especially in high-efficiency equipment.

5. Have Annual HVAC Tune-Ups – A professional can inspect the switch and vacuum lines.

Certain preventive actions can save you from midwinter furnace breakdown and costly repairs.

When to Replace the Pressure Switch

Most pressure switches last 5–10 years, depending on usage and furnace type. Replace yours if:

• It fails continuity tests

• It’s corroded or cracked

• It sticks open repeatedly even after cleaning

• The diaphragm no longer responds to pressure

Replacement switches average $20–$70, and installation is simple for most homeowners with basic tools.

❄️ Furnace Brands with Common “Pressure Switch Stuck Open” Codes

All furnace makers display other error codes when this happens. Here are some examples:

Final Thoughts

A stuck open furnace pressure switch can seem like a critical issue, but most of the time it is caused by simple airflow obstruction or minor component failure. By checking the vents, tubes, and inducer closely, many homeowners can remedy the situation without calling in a professional.

But since the switch is a safety mechanism, never bypass or disable it permanently. In case the problem continues even after basic troubleshooting, it’s advisable to have a professional HVAC technician inspect your furnace to ensure that it operates both safely and efficiently.

Quick Recap

• The pressure switch allows safe venting before ignition.
• “Stuck open” means that it did not close when it was supposed to — which shut down the furnace.
• Typical causes: blocked vent, condensate blockage, faulty switch, or weak inducer.
• Fixes include vent cleaning, clogged tubing clearing, or switch replacement.
• Prevent issues with annual furnace maintenance and clear out air passages.

Get Your Furnace Running Smoothly

Regular maintenance of your pressure switch and air flow system will have you warm all winter, efficiently and safely.

If the furnace pressure switch is stuck in the open position, you can relax — now you know exactly what to do.