Furnace Pressure Switch Stuck Open? Causes and How to Fix It

Is your Furnace Pressure Switch stuck open? This article addresses the possible causes, and how to fix them. Our guide is:

  1. Complete – We introduce you to the pressure switch and familiarize you with its purpose.
  2. Troubleshooting the switch – Sometimes DIY diagnosis is possible, which we’ll explain.
  3. Solutions and costs – There’s more to it than pressure switch replacement cost, because the problem often isn’t with the pressure switch.

Some things that seem very complex are really very simple. Your furnace, and the system that runs it, is a simple concept: burn something to create heat, then pump the heat through the house.

But to make it perform safely is a sophisticated operation requiring many cooperating parts.

Introducing: The Furnace Pressure Switch

Please NOTEIf you are considering doing this work yourself instead of hiring a professional, make very sure you understand your furnace operates using high voltage electricity and extremely flammable gas. If you are not confident in your mechanical knowledge and abilities, you are urged to hire a certified HVAC technician. Our Free Local Quotes option is a quick, convenient option for getting advice, having your questions answered and getting free, competitive estimates from prescreened HVAC pros in your area.

What is a furnace pressure switch? What does it do? Why is the furnace pressure switch stuck open?

What it is: The job of a furnace pressure switch is to monitor pressure within the furnace. The switch is normally in the Open position. When the furnace starts, the first function is the draft inducer motor coming on to create negative pressure which ensures that combustion gases get vented.

What it does: When the switch senses that pressure, it switches to the Closed position, completing an electrical circuit, and allowing the furnace to move to the next stages – firing up the burner and blower to create and circulate heat. If that drop in pressure doesn’t occur, it means the furnace isn’t getting vented, so the switch won’t close, and the furnace won’t fire – or if it has fire, it will shut down immediately.

Think about it. Furnace combustion gases contain deadly carbon dioxide. If the furnace isn’t venting properly, removing those gases, they can and probably will back up into your home if the furnace were to fire up.

Troubleshooting the Furnace Pressure Switch

Be safe! Before performing and do-it-yourself diagnostics or repairs, turn off the power to the furnace using the switch mounted on or near the furnace. It looks like a typical light switch. If you can’t locate it, turn off the furnace breaker in the breaker box. Codes in many places also recommend you lock the electrical box while working on electrical equipment to ensure nobody turns the power on during the work.

Here are common pressure switch problems and how to fix them.

Pressure Switch Problems

Let’s start with the switch itself, but again, it often isn’t the cause of the pressure switch stuck open.

Check the pressure switch: Remove the main access panel and inspect the hoses connected to the furnace pressure switch. They should be in good condition, unobstructed, and connected at both ends.

Solutions: Reconnected loose hoses. Remove debris; replace any cracked hose(s). If the hose ports – where the hoses attach to the switch – are cracked, replace the pressure switch.

DIY Cost: $14 - $44 This is the price range for most gas furnace pressure switches. Average cost is about $25.

Pro Cost: $80 - $200 Cost includes a service call, which starts at about $80 but could be higher where you live. From there, it depends on whether the part is replaced and the cost of the part.

Furnace Lockout

When the thermostat calls for heat, the Integrated Furnace Control unit (IFC) checks the pressure switch. It should be open at this point until it senses negative pressure. If the pressure switch is in a closed position prior to there being negative pressure caused by the inducer motor, the IFC will stop the furnace ignition sequence. If the IFC senses a prolonged or repeated break in the ignition sequence, the IFC may put the furnace into lockout mode. 

Solution: There are two things needed. The first is pressure switch replacement. Secondly, the furnace needs to be reset.

Cost: See the above solution/cost. It’s the same.

Pro Tip: You can usually reset a furnace by turning off the furnace main power breaker, located in the electronic breaker box of your home.

Pro Tip 2: Your furnace might tell you what’s wrong with it by flashing a code – a light or lights blinking in a certain sequence. See your furnace manual for what each code means and what needs to be done. This video discusses Pressure Switch Error Code issues and how to resolve them.

Temporarily Stuck Pressure Switch

If the blower fan will not turn on and you are receiving a pressure switch error code, remove a wire from the pressure switch. If the blower fan turns on, that means the pressure switch is stuck closed.

Solution: A light tap may open a stuck-closed pressure switch, causing the blower to start. This is indicative of a bad pressure switch which has been stuck closed, and it must be replaced with a new Original Equipment Manufacturer (OEM) pressure switch.

Cost: The same as above, $14 to $200, for DIY or pro pressure switch replacement.

Other Causes of Furnace Pressure Switch Problems

If you’re handy, you might try this advanced pressure switch troubleshooting. If what’s said doesn’t make a lot of sense to you, skip it.

To test for a stuck-open pressure switch, recycle the furnace. Remove the wire connections from the pressure switch. When the thermostat calls for heat, the draft fan will start. While the fan runs, short the pressure switch wires (manually connect them with a jumper). If the system starts, there is a problem with furnace pressure but not specifically the pressure switch.

Blocked Air Intake or Exhaust

Your furnace needs fresh air for combustion. Then, combustion gases must be vented. If either the intake or exhaust is blocked, the inducer fan won’t create negative pressure, and the pressure switch will remain open. It isn’t “stuck” open, it’s doing its job.

Common causes of a block intake or vent are bird nests, leaves, wasp nests, snow and ice.

Solution: Check the flue. If you have an 80% furnace, the vent is likely on the roof. If it is a high-efficiency condensing furnace, the vent might be on an outside wall of your home.

Check the air intake too to make sure it is clear and clean. Finally, inspect the air vents on the furnace cabinet, and clean them if they’re blocked.

Cost: $0 if you’re able to do it yourself – and do it safely. The cost for a pro to check for and remove debris ranges from about $80 to $150.

Blocked Condensate Drain

Ninety per cent efficient furnaces have a condensate drain, which may become blocked. Condensation backup can get into the blower fan housing, causing furnace pressure problems.

Solution: We’ve completed a comprehensive guide to Troubleshooting a Blocked Drain Line that includes solutions, equipment needed and a how-to video.

DIY Cost: $0 to $100 depending on whether you need to purchase equipment and the quality of the equipment, such as a shop vacuum, that you select.

Pro Cost: $80 to $275 depending on how long the job takes.

If your furnace is getting older, and repair costs are mounting, consider furnace replacement. Our Furnace Repair Guide talks about your options and how to know when to replace vs repair.

Or see our Gas Furnace Reviews and Buying Guide for comprehensive information on brands, furnace performance, efficiency options and much more.

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