Furnace Pressure Switch Stuck Open/Closed: The Troubleshooting Guide

A furnace pressure switch is a safety device that keeps your furnace working only when venting is safe. It checks for negative pressure made by the inducer motor to confirm that dangerous combustion gasses can leave the system properly. 

If the venting is blocked or not working right, the pressure switch stops the furnace from running to prevent any risks.

In most furnaces, a pressure switch stuck closed is indicated by two blinking lights on the control board. Three blinks usually mean the pressure switch failed to close properly.

In this article, we’ll explain what it means when the furnace pressure switch is stuck open or closed. You’ll learn the signs of each issue, why they happen, and a simple guide to troubleshooting them. 

Pressure Switch Stuck Open

When your furnace’s pressure switch is stuck open, it means the switch isn’t detecting the right pressure to allow the furnace to start. In other words, the switch remains in the “off” position, blocking the furnace from igniting. And unlike a simple reset button, you can’t just reset this switch manually.

It’s a safety feature that needs the right pressure conditions to work, so you’ll have to find and fix the underlying issue to get things running again.

Here’s what might be causing it and some straightforward solutions you can try.

Reasons Your Pressure Switch Stuck Open

1. Blocked Vent Pipe

One of the main reasons a pressure switch won’t close is a blockage in the vent pipe. The vent pipe carries exhaust gases, and if it’s clogged with debris, snow, or even a bird’s nest, it can disrupt the airflow. When the air can’t move freely, the pressure switch won’t get the negative pressure it needs to close.

It can also cause the system to use more energy as it struggles to maintain airflow, leading to higher electric bills.

Solution: Check the vent pipe for any visible obstructions and clear out any debris or blockages. During winter, make sure snow or ice isn’t covering the vent. It’s also a good idea to trim any trees or shrubs growing near the vent to keep it clear.

2. Faulty Inducer Motor

The inducer motor creates the negative pressure that activates the pressure switch. If the motor fails or struggles to start, it won’t generate enough airflow, keeping the pressure switch open.3201

Solution: Listen to your furnace. If the inducer motor sounds like it’s struggling or isn’t turning on at all, it could be a sign of failure. You may need to have a technician inspect or replace the motor. Sometimes, it’s just a matter of cleaning the motor or replacing a worn-out part.

3. Clogged Inducer Port or Hose

Over time, the inducer port or hose can get clogged with dust or moisture buildup. When these passages are blocked, the inducer motor can’t create the right pressure to close the switch.

Solution: Inspect the inducer hose and port for any blockages. Carefully disconnect the hose and blow through it to clear out any clogs. If there’s moisture inside the hose, dry it out before reconnecting it. This quick fix can often solve the problem and get the furnace running again.

4. Low Voltage or Electrical Issues

The pressure switch relies on a steady flow of electricity to work. If there’s low voltage or an electrical issue, the switch may not close properly. Loose wiring or a malfunctioning circuit board could be the culprit.

Solution: Start by checking for any loose or disconnected wires near the pressure switch. If you’re comfortable with electrical work, use a multimeter to check for proper voltage (it should measure approximately 24 to 28 volts AC across its terminals). However, if you’re unsure, it’s best to call a technician to inspect the wiring and circuit board for any issues.

5. Faulty Pressure Switch

Sometimes, the issue isn’t external—it’s the pressure switch itself. The switch can wear out over time or get damaged like any other part. A faulty switch won’t close, even if everything else is working fine.

Solution: If you’ve ruled out other causes, the switch itself might need replacing. Pressure switches are affordable and relatively easy to replace (priced between approximately $60 and $200 CAD, depending on the brand and model). Brands like Lennox and Carrier offer reliable options, but it’s essential to choose a switch that matches your specific furnace model for the best performance.

If you’re not familiar with furnace repairs, it’s safer to have a technician handle it.

An OEM (Original Equipment Manufacturer) pressure switch is recommended to ensure compatibility and maintain the furnace’s safety features.

Pressure Switch Stuck Closed

When your furnace’s pressure switch is stuck closed, the switch signals the furnace to keep running even if there’s a venting issue or a lack of proper airflow. This situation can be risky because the furnace may run without proper exhaust.

Reasons Your Pressure Switch Stuck Closed

1. Faulty Pressure Switch Contacts

Over time, the contacts inside the pressure switch can wear out, get stuck, or simply malfunction. When this happens, the switch may stay in the “closed” position, allowing the furnace to run continuously without proper checks.

Solution: If the pressure switch is old or has shown signs of wear, consider replacing it. Pressure switches aren’t too expensive, and a new one can often solve the issue right away. Replacing the switch is a straightforward job, but if you’re not comfortable doing it yourself, a technician can handle it.

2. Excessive Moisture or Water in the Switch

Rainy or humid weather can cause moisture to build up around the pressure switch, which sometimes makes it stick. 

This also often happens if there’s condensation inside the furnace, which can drip into the switch and affect its operation.

Solution: Check the area around the pressure switch for any signs of water or condensation. If you find moisture, dry the switch and surrounding areas carefully. It’s also wise to inspect the furnace for any leaks or areas prone to condensation to prevent this from happening again. In some cases, installing a condensate trap may help keep moisture away from the switch.

3. Blocked Condensate Drain Line

If your furnace is a high-efficiency model, it likely has a condensate drain line that removes excess water produced during operation. When this drain line gets blocked, water can back up and interfere with the pressure switch, making it stick in the closed position.

Solution: Clear any blockages in the condensate drain line. You can usually do this by disconnecting the line and flushing it with water or using a small brush to clear out debris. If you notice frequent clogs, regular line cleaning can help prevent the issue from returning.

4. Debris or Dirt in the Switch

Debris, dust, or dirt can collect inside the pressure switch over time, causing it to stick in a closed position. This buildup affects the switch’s ability to function properly and can make it “think” the furnace is safe to run when it isn’t.

Solution: Disconnect the pressure switch and inspect it for any dust or debris. Gently clean the switch with a soft brush or compressed air, being careful not to damage any internal parts. 

5. Electrical Malfunction

If there’s an issue with the furnace’s electrical system, it can cause the pressure switch to stay closed. This might include issues with wiring, the control board, or even a short circuit within the switch.

Solution: Inspect the wiring connected to the pressure switch and the control board for any loose or damaged wires. A multimeter can help verify the voltage if you’re familiar with electrical work. If you’re not, it’s safer to contact a technician who can troubleshoot electrical problems and repair any faulty components.

Pressure Switch Open with Inducer On

The inducer motor’s job is to pull out combustion gasses, creating the airflow needed to close the pressure switch. If the switch doesn’t close, it means the furnace isn’t getting the right airflow to run safely.

This can happen for a few reasons. First, there might be a blockage in the vent pipe—like debris or a bird’s nest—that’s stopping the airflow. Second, the inducer motor could be weak or not working right, so it’s not pulling enough air. Lastly, there could be a leak in the system, letting air escape and messing up the pressure.

To fix this, you’ll need to check the vent pipe for blockages and make sure the inducer motor is working properly. Also, look for any leaks in the system. Regular cleaning and maintenance of these components can help prevent this problem from happening again.

When to Call a Professional

While some furnace issues can be handled with basic troubleshooting, certain situations require the expertise of a professional technician. 

Here are some signs to look for to know when to call an HVAC professional:

• Repeated Pressure Switch Issues: If you’ve tried cleaning and checking for blockages, but the switch keeps getting stuck, a professional can help diagnose deeper problems.
• Inducer Motor Not Working Properly: If the inducer motor isn’t running or is struggling to start, this can affect the pressure switch. A technician can determine if it’s the motor, wiring, or something else.
• Electrical Issues: Loose wiring or problems with the control board aren’t safe to tackle without experience. Call a pro if you suspect electrical trouble.
• Water or Moisture Around the Furnace: If you notice moisture around the pressure switch or in the venting, it could mean there’s an issue with drainage or condensation that needs expert attention.
• Frequent Furnace Shut-Offs: If the furnace keeps shutting off unexpectedly, the pressure switch could be the issue—or it could be a sign of other underlying problems.

Regular Maintenance Tips for Your Furnace Pressure Switch

A furnace pressure switch ensures safe operation by regulating airflow, which prevents overheating and keeps the combustion process efficient. 

With regular maintenance, furnace pressure switches can last around 10 to 15 years. Here are some tips to keep your pressure switch functioning properly:

• Inspect the Vent Pipe Regularly: Check for any blockages or debris in the vent pipe that could affect airflow and pressure. Clear any obstructions to maintain proper ventilation.
• Clean the Pressure Switch and Surrounding Area: Dust and dirt can accumulate on the pressure switch and nearby components. Gently clean these furnace areas to prevent buildup from interfering with the switch’s operation.
• Check the Inducer Motor Functionality: Ensure the inducer motor runs smoothly, as it’s crucial for creating the necessary pressure differential. Listen for unusual noises or vibrations that might indicate a problem.
• Inspect Electrical Connections: Look for loose or corroded wires connected to the pressure switch. Secure any loose connections and replace any damaged wiring to ensure reliable operation.
• Test the Pressure Switch Periodically: Use a multimeter to check the continuity of the pressure switch. This helps verify that the switch is opening and closing as it should.
• Schedule Professional Maintenance Annually: Have a qualified technician perform a comprehensive inspection of your furnace, including the pressure switch, at least once a year. They can identify and address issues that are not apparent during routine checks.

Conclusion

Fixing a furnace pressure switch is one of those jobs that sounds simple but gets tricky pretty fast. Most homeowners aren’t looking to mess around with pressure switches, and who can blame them? 

It’s difficult, and you don’t want to risk worsening things. But finding a pro who knows their stuff can be tough too. You want someone who’ll do it right without adding stress to your day.

If you’re done with the hassle and want a quick, reliable fix, getting a trusted HVAC contractor is your best bet. Grab a free quote from top HVAC contractors near you today! 

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If you’re experiencing common furnace issues, troubleshooting them yourself might save you time and money. For practical solutions to some of the most frequently encountered furnace problems, check out our Furnace Troubleshooting Guide. This comprehensive guide covers everything from inadequate heating to strange noises, helping you identify potential issues before calling for professional help.