How To Test a Gas Furnace Pressure Switch

By | October 26, 2015

What is a gas furnace pressure switch?
A furnace pressure switch is there to tell the furnaces control board that the inducer motor is working.

An inducer motor spins and pulls out toxic gasses that come from burning gas and pushes it all out a vent creating a vacuum.
gas furnace pressure switch inducer motor
 
Before natural gas can go into a furnace and start a flame, it uses the pressure switch as a safety to know that the gasses are being vented.
 




 
How does it work?
A pressure switch has a diaphragm that pulls out when a vacuum is applied to it. If there is no vacuum the diaphragm doesn’t pull out.

The diaphragm uses a switch connected to it that will show open or closed depending on what the diaphragm is doing.
 
When there is a vacuum, the pressure switch diaphragm pulls out and closes the switch.
 
When there is no vacuum, the pressure switch doesn’t move and the switch stays open.
 
Example gas furnace pressure switch on Amazon
LEFOO LF31 air Furnace Differential Pressure Switch Pressure 100pa used in heating ventilation refrigeration cooling heater

 
How do you test the gas furnace pressure switch?
To test the pressure switch we need to use a multi meter and measure the continuity of the switch as it opens and closes.
 
A multimeter will be needed to test the switch for continuity. Basically The switch needs to be tested to see if it is closing when the pressure diaphragm pulls in.
 
To do this the pressure switch wires are removed and the furnace turned on, Using a meter to test it, if the switch closes than it is good, if it does not than something is wrong.
 




 
Take the two wires off the pressure switch and touch them with the multimeter leads. The multimeter should be set to read continuity.
 
Sorry about the grainy pictures I will get better ones when I can.
 
Take the two wires off the pressure switch.
multimeter testing pressure switch
 
The two wires have been removed.
test a gas furnace  pressure switch
 
Turn the furnace on and the draft inducer motor will turn on and begin to blow the air out the vent creating a vacuum that pulls in on the diaphragm and closes the switch. The multimeter will see if there is any change and if the switch closes or not.
 
Switch is open no change in continuity
multi meter to test a gas furnace
 
Switch has closed showing continuity and is good.
How do you test the gas furnace pressure switch
 
Where to buy a gas furnace pressure switch
HVAC supply stores will sell you a pressure switch but usually at elevated prices. If you can wait a few days I would buy from Amazon since they have a wide selection.
 
Example Pressure Switch On Amazon.
LEFOO LF31 Furnace Air Differential Pressure Switch-Fall Pressure 500pa

 
If there is no change then there is a problem with the switch. Keep in mind that a kinked hose, clogged flue pipe, or anything that stops the inducer motor from pushing air out the vent will cause a good pressure switch not to work.

 

 




 

23 thoughts on “How To Test a Gas Furnace Pressure Switch

  1. Vernon kruger

    Have a rheem Corsair down flo furnace it lites burns 5-8 seconds shuts off have replaced flame sensor

    Reply
  2. Christopher

    Having a no igniting issue on my furnace pressure switches have all 26 volts to them but once it clicks to get hot surface to lite 0 volts show on other side of the pressure switch. So thinking that pressure switch is bad. Trying to see if jumping will get me temporary heat though the night.

    Reply
  3. Joseph

    I have a rheem that is being funky. It will run fine for a few hours then shut down. I have found that if I go “wiggle”the leads on the pressure control the inducer will kick up and work for a few more hours then stop. I’m not a pro but I believe it’s the pressure control contacts being bad. Can I clean them with a Emory board or just replace the whole switch.

    Reply
  4. Ryan

    My Lennox pulse 21 furnace the inducer motor wont turn off. I think it could be a pressure switch issue. Where do.i begin looking?

    Reply
    1. Andrew

      I simply blew into the tube of both pressure switches several times to free whatever was sticking inside, and it worked. Next time I might squirt in some silicon into the switch and pump it back and forth to distribute lubricant where needed. WD40 probably should not be used since it might affect the rubber hoses, probably not Neoprene.

      Reply
  5. don suttles

    Hello I have a York furnace and I’m getting a code for limit switch open.one hvac guy said my furnace was fine.the let her said it was the limit switch.so I went out bought one and installed it.it did nothing to change the problem.I’m thinking its the pressure switch.I heard you can test it by pulling line off of inducer blower and if you suck it should click and if you blow it should click.is that true?my furnace will kick on put heat out for a little then the air gets cold and furnace will run till I turn thermostat off.then if I wait 5 mins it will work ok for a few then back to same thing

    Reply
    1. Andrew

      Puffing into the switch tubes several times freed the sticky parts inside, I felt the membrane moving, so gave it few more puffs and the furnace took off flawlessly It is 123f right now. Calling for service Xmas night would cost quite bit, It took me about an hour to figure it and fix it all. It 12:30 night and the home is warm again. It helps to have some tools handy. Andrew

      Reply
  6. Maurice Hill

    Hello everybody. I have a carrier furnace problem. My furnace will not run unless i unplug the hose connected to the pressure switch. Can anyone help?

    Reply
  7. Kevin Morris

    I replaced the control board on my goodman GMT 090-4 the furnace worked for about a day then gave a code 3- blinks the pressure switch I replaced that worked for a few hrs now when I call for heat nothing happens no inducer nothing and I still get the code 3-blinks I’m at a loss as to what to do next

    Reply
  8. Luke

    Don’t blow or suck on your pressure switches, people. They are delicate and you could damage the diaphragm in them. And Pete is right. These switches are dialed in for a certain pressure specific to that model furnace. Adjusting them is dangerous, allowing the furnace to run when something is wrong with it. The pressure switch is usually the symptom and not the problem. Find a contractor you trust and get on a plan. Repairs will be cheaper that way and they can keep an eye on your equipment.

    Reply
    1. Andrew

      Luke, in the middle of Xmas night, puffing worked. In addition , now I know what to do next time. The switches were the problem and it leaves me in charge rather than being a helpless consumer.

      Reply
  9. Mark

    Not sure if anyone is watching.
    My daughter has a Trane XE90. I am getting the 3 blinks on the circuit board. The hose from the bottom left coming out of the unit (I am a handy electronics repairman and repaired/installed medical equipment for 30 years, but have no HVAC lingo in my forte ;), nxt to about a 3/4″ diameter hose, connects to the pressure switch (with a “PO4” yellow label on it), and the hose then T’s to the burner assy area and the gas valve.

    If I pull the hose from the bottom left (that goes into the back of the pressure diaphragm) I am getting pressure – not vacuum. So I am thinking the inductor motor may have a blocked vent (they did have bee’s in one of their roof vents last year that I found by accident patching around some flashing. Fortunately they didn’t attack; I have allergies).

    That said, I “tested” the switch by sucking and blowing into the hose (that again, goes into the back of the switch assembly from the lower left of the unit). Without a lot of blwoing I could hear it escaping from the switch assembly. Blowing would get the switch to “click”, but upon not blowing it would not “click” back. However, if I then sucked, it would click… but upon stopping the suction i t would again, not click. I am thinking the diaphragm has a tear and is hanging up in there.

    I know it is not common to have two issues at once, but after 30 years of repairing, I know it can happen and is a pain in the but to figure out!

    Thanks for the help
    Mark

    Reply
  10. Theresa

    I have checked everything and my furnace did run…I may need a new draft enducer…I cleared my humidifier hoses and now my furnace will not come on… I removed hoses from pressure switches before clearing hoses…when my furnace did run I would get a pressure switch stuck open fault and it would shut down after a minute.

    Reply
  11. Kevin beatty

    I need paperwork on a universal pressure swith so I can a just it I accidentally threw mine away

    Reply
    1. jim craft

      Here’s my recent Thanksgiving experience fixing a Goodman Air Pressure Switch B1370158.

      My 89-year old Father-in-law called to convey that his house is 45 degrees (after a day; those Purple Heart guys bear up well). I check the obvious stuff then take off the Everrest furnace doors and the 3 “blinks” code points to the pressure switch/flue problem. Flue is clear so I call Taylor Heating. They respond within an hour and the nice young man checks for switch continuity (none) and jumps it to warm the house temporarily. Given the holiday, he quotes us $303 for the part, $285 to open the supplier, $216 acquisition fee, and $60 travel. We thank him and pay the $140 service call fee, thinking Dad could stay at the Hilton for a few days for less $$$.

      So I play with the switch and note that it closes when a vacuum is applied to (suck on) the port. So either the contacts are bad or the spring tension needs adjustment. I pried the wire prongs-access door off; the contacts looked fine and noticed a movable small cylinder in the center of the switch. This controls spring tension/needed vacuum and is adjustable by an exterior threaded square drive plastic set-screw (used a small screwdriver). Lacking a vacuum pump, I adjusted it to the lowest vacuum (inhales) to make contact without slop/looseness. One screw and a hose connection (10 seconds) later, it surprisingly it worked. Seems that any loss of vacuum like a loose or cracked hose ($1) could also cause problems.

      There’s no better feeling than tinkering/fixing something esp, when other are depending on you but I may just order a new switch ($17) from Amazon and pocket the 1500% markup.

      Reply
      1. pete astacio

        It is always expensive to call out a service company on a holiday weekend. However…. I’d like to see a copy of the invoice that he gave you. Sounds to me like he was either trying to rob you or your exaggerating the price enormously!! Even on a holiday weekend it’s not going cost you almost $1,000 for that part. I’d say that depending on the company it should cost around $450.00 – $600.00 though. Also that is not marking the part up by 750%. It’s called the cost of doing business. Truck takes gas, Employee is at least at time and a half probably at double time on a holiday weekend. On top of that they charge you the $140.00 service call because a lot of people try to do just what you did call a company out to tell you what is wrong with it and then you try to half ass fix it your self. If the furnace hasn’t shut off again by now I’m sure it will soon. What you did to the switch could “possibly” fill your house with Carbon monoxide. What if a bird gets in there but does not block it up all the way. You might as well of just left the jumper on. Not to mention the issue with ordering a pressure switch on Amazon. LOL, you void the warranty on your furnace by putting a non factory pressure switch and of course there is always the safety issue witch is the most important issue… I would of called another company and let the professionals do it just my opinion though… SAFTY FIRST!!!!!

        Reply
  12. John

    When a pressure switch opens and shuts off the burner, there are basically two things that could be wrong 1… the pressure switch is defective and needs to be replaced or 2… the pressure switch shut off because of a defect in the venting system that is not creating a vacuum.

    Too many repairman tend to jump to the conclusion that the pressure switch is bad and replace it, and indeed pressure switches CAN be bad. But about 90% of the time the pressure switch is opening because it is detecting a defect in the venting system which needs to be corrected for the furnace to operate safely.

    Reply
    1. Jim Edwards

      Help! I have a Goodman unit. I tested the GFS as per above and the continuity did not change. I order and received one from Amazon. Install it and I still get the 3 blinking lights. I removed the skinny hose from the induction motor and applied about 20 PSI of air, no apparent leakage and the hose appears to be in good shape. The larger rubber hose going to the venting system also appears to be in good shape. I removed it thinking that if the vent was obstructed, the unit would work because it was no longer blocked. am I missing something else? Thanks. Jim

      Reply
      1. Aaron Benetti Post author

        The switch needs to show a continuity change. Is the draft inducer motor turning on to create a vacuum? Basically there should be a vacuum that closes the switch so either there is no vacuum or something is blocking it.

        Reply

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