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Problem: My furnace’s ignitor does not glow? This could be a pressure switch, limit, roll out switch, or furnace control board problem.

I see this problem many times during the heating season here in Louisville, KY. When your thermostat calls for heat the draft inducer (please see pictures below) should start which creates a draft in your vent pipe. If your vent piping is open and your pressure switch is working properly the pressure switch will close the connection between two wires and send a signal to the control board saying, “Yes it is OK to continue with the ignition process.”



Above are pictures of a draft inducers showing where the pressure switch tube attaches. Pressure switch tubes sometimes attach on the top bottom or near the center of the draft inducer. These are just two of hundreds of different draft inducer designs. Some are metal and some are plastic. Most of the time draft inducers are plastic on 92% furnaces & metal on 80% furnaces. Make sure the opening where the pressure switch tube is attached to the draft inducer is open. Sometimes a small wire can be used to reestablish the hole. Make sure your water drain on a 92% furnace is open. If too much water backs up into the draft inducer it will not allow the pressure switch to close.

Also, all of your limit controls and other safety devices must be closed (a closed circuit between the two connections) to allow the furnace ignitor to glow and start the ignition process. If there is a break down in the pressure switch or other limit safety controls the furnace, for your safety, will not proceed with the ignition sequence. The computer chip inside the board says, Wooooh, something is wrong here! Most furnaces will try this sequence for a total of three times then lock out. On most furnaces the only way to get them out of lock out mode is to turn the switch (looks like a light switch) on the side of the furnace to off and then back on again.

Above is a picture of three different types of limit controls. The two on the ends should reset automatically when the furnace cools down. The rollout limit switch in the center has to be manually reset by pressing the button in on the top. All limit controls are there for your safety. Never by-pass these.


How do you test a pressure switch and other limit controls?

This is for people who are experienced with electrical equipment and the use of a Volt Ohm meter. *Please never by pass a pressure switch or limit control. The pressure switch and limit switches are there for your safety. In the top picture I am testing a rollout limit switch to see if it is good. You would test by setting your volt meter to “Volts AC” and test the rollout switch by placing one meter probe on one terminal of the rollout switch and your other meter probe to a good ground. Below I have the red probe touching the top terminal of the rollout switch and the black probe touching a ground (body) of the furnace. You can see the I am getting 25.85 volts which means the rollout is good on the top terminal. I would next test the bottom terminal of the rollout by touching the red meter probe to the bottom terminal on the rollout switch and the black terminal to ground. If I get 25.85 volts on the bottom terminal the rollout is good. If I do not get any voltage on the bottom terminal then the rollout has tripped and can be reset (if equipped) by pressing in on the little button or replaced. If your rollout switch is tripped you probably have a stopped up heat exchanger or a leaking heat exchanger. I would recommend calling in a service technician to find out why the rollout switch tripped. If you have an open limit control either the furnace has over heated or the limit has gone bad. Problems that would make a limit open up would be dirty filters, dirty evaporator coil causing a restriction in the air flow or a slow blower motor (check the capacitor on the blower motor to make sure it is good). A weak blower motor capacitor will cause the blower to run slow and eventually fail. Pressure switches, and limit switches can be tested with a meter in the same manner. With the furnace calling for heat you can test each of the terminals on the pressure switch to ground to make sure the pressure switch is closed. You should be getting between 24 to 28 volts from each terminal to ground if the pressure switch is closed. If the pressure switch is open you either have a stopped up vent, drain line (if you have a condensing furnace) or bad pressure switch. Below we have three pictures of pressure switches. The picture on the left shows the full view of one pressure switch. Please keep in mind there are many different types. The picture on the right shows the two terminals that you can use to test to see if the pressure switch is operating properly. The front terminal is burnt and discolored. This is a clue that this pressure switch has a problem. The picture on the bottom is a black Goodman, Janitrol plastic pressure switch.


Above pic shows me testing a rollout switch.
Above are two pictures of pressure switches. The pressure switch on the left came off a 92%
furnace. It has two connections for pressure tubing. One tube would go to the water collection
box and the other tube would go to the draft inducer. The switch on the right has a burnt front terminal.
Above is a picture of a Janitrol Goodman pressure switch.


How do I test to see if a pressure switch is operating properly?

Set your meter to volts AC, turn your furnace on so it is calling for heat. The draft inducer should start. There should be either two or three wires going into your pressure switch. Probe one lead of the pressure with one lead of your meter while touching the other lead of your meter to ground. Ground would be any bare metal part of your furnace. This must be bare metal. I always try to touch the other lead of my meter to the furnace’s switch box. If you have a two wire pressure switch you should be getting 24 or more volts between both leads to ground. By this I mean if you touch one terminal of the pressure switch with one lead of your meter, and touch the other lead of your meter to a ground, bare metal part of your furnace, you should get 24 or more volts (24 to 28 volts). If you do not get 24 or more volts with the furnace running then you have a pressure switch problem. Your vent could be stopped up, The tube that runs from your pressure switch to the draft inducer could be plugged up or the draft inducer hole could be plugged up. On high efficiency condensing furnaces the water drain line could be stopped up, causing a water back up and blockage in the pressure switch tube. I usually take the drain hose loose from the condensing furnace and use a wet vacuum to open the drain back up. I hope this will help you in troubleshooting your gas furnace pressure switch. If the pressure switch, limit and rollout switches are all closed when the furnace is calling for heat then this could be a bad ignitor. We sell many different types of ignitors for many different brand furnaces on the following page: Please click here to see the furnace hot surface ignitors we sell. Please send us your furnace’s model number if you would like for us to find the right parts for your furnace. Our email address is:



8 thoughts on “Problem: My furnace’s ignitor does not glow? This could be a pressure switch, limit, roll out switch, or furnace control board problem.

  1. Is it possible to test the ignitor relay on the control board or does one replace the entire board? My ignitor will only glow on the 3rd or 4th ignition cycle, and over night when the thermostat is set back, we sometimes wake to a cold house. The thermostat is fine, all switches are operating normally. The ignitor is fine but only gets 120 volts on around the 3rd ignition attempt. The wiring from the control board to the ignitor is fine too. Could it be a cold or broken solder joint from 10 years of vibration? How does one check the relay or is there something else I should look for?

    1. This sounds like a bad ignitor relay on the control board just like you have concluded. The only way that I know to test it is by what you have already been doing and that is test the voltage going to the ignitor. Yes, sometimes the solder joints become loose from vibration or the contacts inside the relay become pitted and wear out. I am not electronically inclined where I could replace parts on the control board. I always replaced the entire control board. If you would like for me to look up parts then please send me your furnaces make and model number. Steve

  2. I have a Lennox gas furnace that is not heating. Igniter is new and although I have 118 ac at the connector it doesn’t glow?

    1. Hi Doug! Maybe you have a bad ignitor relay on the control board. I would suggest trying to test the voltage going to the ignitor when the ignitor (load) is plugged in. If the voltage is under 110 volts with the ignitor plugged in then you might have a bad relay on the control board or an open safety control like a roll-out, limit or pressure switch. I would suggest reading the ohms on the ignitor. A good ignitor will read from 40 to 90 ohms. I hope you can easily find and fix the problem. Steve

  3. And for those of us who have NO experience with furnaces, it might just need to be rebooted like a computer to start fresh; after all, there’s a circuit board inside! Both of mine were blowing air, but no heat, and this simple procedure won’t cause any harm. (1) open the cover; (2) if you don’t see a glow, the pilot light isn’t functioning and there will be no heat; (3) follow the power cord to the outlet, where there’s usually a light switch; (4) turn off the switch and wait for a few minutes before turning it back on; (4) this gives the furnace a chance to reset (reboot) itself and attempt to light the pilot itself. Worked for me, and no service call!! (If there isn’t a switch, don’t just unplug/replug the cord; you’ll want to turn it off at the breaker, wait for a few minutes, and turn on the breaker again.)

    1. Thanks you so very much for your advice! Great idea!

  4. My furnace operates perfectly if it has been off or is cool. After the first cycle, the ignitor does not appear to glow. If I allow it to sit for 20 minutes, then it fires right back up. Not sure why it ignites and burns sometimes but not others.

    1. Hi! Most of the time when the ignitor is erratic in operation it is caused by a control board relay that controls the ignitor contacts not making contact all the time and requires a new control board. If the furnace operates for a while, then shuts off and will not ignite again for 20 minutes, then I would suggest checking the limit and rollout switches to make sure all of them are staying closed. An open limit or thermostatic rollout will not allow the furnace ignitor to glow until the furnace cools down. We have troubleshooting page that shows how to test a rollout. The limit would be tested the same way. Here is a link to our troubleshooting page: If you find an open limit or rollout then you probably have an air flow problem (dirty filter, stopped up evaporator coil, slow blower, etc.) or a stopped up heat exchanger or a leaking heat exchanger. I hope you can easily find and fix the problem. Steve

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