Symptom – Burner lights off, but goes out quickly.
What is it? – This symptom is often caused by a dirty flame sensor that can be easily cleaned. Not all furnaces have a flame sensor, but if your furnace has one, it will be mounted close to the burner with the metal rod positioned in the flame. Flame sensors vary in size (2 to 4 inches) and shape (see examples below). They are powered by a single wire carrying a small amount of electricity (less than 10 DC microamps).
What is happening? – Over time, a coating can form on the metal rod of the flame sensor that is difficult to see. Because of this coating, electricity flow is restricted and the furnace control system “thinks” the burner has failed to light and shuts off the gas as a safety measure even though everything has worked as designed. Flame sensors rarely fail. Broken or cracked ceramic are signs that the flame sensor is defective.
The repair
- As always, turn off all electrical power to the furnace while you are performing this repair. Remove the flame sensor.
- Steel wool or a fine sand paper can be used to “polish” (clean) the sensor rod and remove the coating.
- After cleaning is complete, replace the flame sensor.
- Turn the power back on and check the furnace for proper operation.
Note – If this repair does not restore normal operation call for free diagnosis assistance or bring the flame sensor in to the U-FIX-IT Appliance Parts store for free testing and inspection.
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More From Jim Plummer
Jim is an ex-Air Force pilot and a business school graduate of University of Texas at Austin. He has been in the appliance parts business since 1975.
My gas electric start furnace keeps going out. My husband was a utility Forman in a gas company in IL and hes passed away at 55. So I’ve been trying to work on the furnace myself. Which I have been getting it going. The next day it will be out again. I always change the filter and I’ve been cleaning it when its turned off. 5 years ago it went out. One of his coworkers put the tiniest part in saying you dont want to touch or it might not work. He gently put it in but I cant remember what it is! Thank you! Kim
Hello Kim,
I cannot be certain which part was replaced from your description. Ceramic igniters are the most delicate part on a gas furnace. Give us a call at 817 472-7740 for free diagnosis assistance and we will help you determine why your furnace is not working. Thank you for your inquiry and have a Happy New Year!.
Jim
Thanks for your advice saved 400 dollars. Thanks alot
Darwin,
Wow, It is great to hear about a win like that. Have a Happy New Year!
Jim
How many times can a flame sensor be cleaned before it needs to be replaced,mine has been cleaned twice during yearly maintenance check done buy certified furnace tech. Where can i order one to have on hand in case of an emergency. My furnace is a Trane and 10 years old. and I live in Minnesota and might be a good idea to have an extra one on hand. The model number of my furnace is xv95 model number tuh2b080a9v3va
Thanks Darwin
Hello Darwin,
Sensors go bad when the ceramic insulator cracks, otherwise they can be sanded endlessly. It would be easier to believe if the coating we sand off was visible in some way. The low voltage that is transiting the flame sensor through the flame is easily blocked by the invisible residue that builds up on the sensor. The vast majority of flame sensors we sell replace good sensors in spite of us recommending against it. There are no internal components, it is a metal rod mounted in a ceramic insulator.
When sanding the insulator does not correct the problem, many people will replace it, “just to make sure,” before looking for the malfunction.
I am happy to sell you a standby replacement sensor if you want one. I don’t recommend it, because it almost always needs light sanding and it almost never needs replacing. You are much more likely to need a safety limit, a pressure switch, a capacitor, a draft inducer motor or a blower motor over the life of your furnace. Call us at 817 472-7740 if you would like us to ship you one. Thank you for your inquiry.
Jim
I don’t think my furnace has a flame sensor ride if so I cannot find it
Christopher,
Call the 817 472-7740 for free personalized help in diagnosing your symptoms. Have your model number when you call. Thank you for your inquiry.
Jim
My flame sensor has been cleaned a few times over the last seven years but this last incident needed replacing. Before replacing the flame sensor my furnace would try to turn on in intervals of three.
With flame sensor replaced the furnace works but now repeatedly turns on and fulfills three cycles (same interval anount)
Broken – Tried to ignite in threes.
Replaced – Ignites in threes.
Hello Melissa,
I am not certain what your symptoms are from your description. Please call the nearest U-FIX-IT Appliance Parts for personalized diagnosis assistance. Thank you for your inquiry.
Jim
My furnace was on but won’t cut off, what is this problem. I only can cut it off by the switch on the wall. I guess the control switch. What makes it not go off and is this expensive. How much please?
Hello Denise,
You have not given enough information that I can be of help. You can call U-FIX-IT Appliance Parts at 817 472-7740 for free personalized help in diagnosing your furnace.
Thank you for your inquiry.
Jim
I have a York furnace, keeps shutting on and off..fan is on, flame comes on, replaced flame sensor..still the same problem..
Ed,
It is exhibiting the classic symptoms of a flame sensor problem. I would make certain the flame is impinging on the flame sensor. I would check the furnace control board has a good ground. I would check all the safeties connected to the control board. If everything checks good, that would lead me to believe the control board is bad.
Call the nearest U-FIX-IT store for personalized free troubleshooting assistance.
Jim
My gas furnace would cycle endlessly and not start. The hot igniter would come on, the exhaust blower would come on, then the main burner gas would come out and ignite, then about 2 seconds later, it would go out, shut down intentionally by some valve. Then the cycle would start again.
I took out what must have been this flame sensor, and, even though it did not look very dirty, brightened it up a little with a green scotchbrite pad, wiped it off, then put it back.
Now my gas furnace is running. Maybe I fixed it?
Steve,
Yes, It sounds like you did fix it. Flame sensors acquire an invisible coating over time which can block the flow of a very low voltage from the sensor through the flame and back to the control board. The board shuts off the gas whenever it does not detect the voltage. Good job!
Jim
OK. Jim
How about this I have an Air Pro bgu10016a ( coleman ) and have just recantly replaced the main fan motor. After the succsesful repalcement, the furnce would light then go out also showing the false flame sensor code. “Simple Ill clean it” or so I thought, cleaning the sensor was no help. Now I have a new flame sensor and the same old problem. So what would you say is my next coures of action short of an axe should be?
David G.
You have been doing the things that make sense. If it was my furnace, I would check that all the safeties have continuity, the flame is impinging on the flame sensor, and I would make certain the control board has a good ground. If all those check out, then I would replace the control board. Call the nearest U-FIX-IT store if you have questions.
Jim
My furnace ignites and instantly goes out it cycles 3 times before stopping completely… cleaned sensor still not working even changed air filter please help no money 66 degrees now and dropping
Mike,
I would make certain the control board has a good ground and then I would locate the wiring diagram on the indoor unit. Furnace control boards often have flashing lights that indicate error codes which are explained on the wiring diagram. If there are no error codes to follow, I would find the safeties shown on the diagram and check them for continuity. Any open safety is bad and should be replaced. You can take your wiring diagram to the nearest U-FIX-IT store for personalized help on how to proceed with the troubleshooting.
Jim
We have Benton bg-450 burner..its tripping in every 30-40 minutes with error code of 7 which shown in manual that problem may be in fuel valve,electode is grounded or bad setting of burner.I have replaced electrodes and controller and checked ground.But problem not solved…please help
Singhm,
Unfortunately, I have no answer for you. Oil burners are not a product commonly sold here in Texas. Thank you for your inquiry.
Jim
I have two boys in college right now very expensive and money is tight . I read about how to fix this problem as my furnace would light but wouldn’t stay lit. OMG THANK YOU THANK YOU THANK YOU saved me a lot of money!!!!!!!! Have a great day!
Thanks Jim!!
I got home tonight and my thermostat read 58 degrees when it was set to 67. I tried resetting it at the fuse box to no avail. I came across your helpful article and with only removing 3 screws and about 10 minutes time my heater is working again! You saved me a lot of stress, time and money.
Thank you so much!
Jeffy,
Glad to hear we helped you.
Thank you for letting us know.
Jim
Last summer I had a leak in the ac condensate tray. It caused the fan to turn on randomly but the ac always worked. In the fall it was time for heat and nothing. Not even the draft motor would come on. I replaced the board only to find that the problem still existed. A replacement of the gas valve corrected the issue for a while.
I now have chronic random instances of the burner either not lighting or lighting and immediately shutting down. Sometimes If I cycle power, or even remove the igniter and flame sensor and then reinstall (no cleaning) the systems works fine for a while. Sometimes hours, sometimes days but has always come back. I am stumped. I have even added second ground lines to see if that would help. No change so I removed it. Any advice?
Mark,
Random malfunctions are the most difficult to troubleshoot and there is no easy answer here. Troubleshooting can only take place while the system is working incorrectly. Use the wiring diagram and a volt meter to start the process. Is the low voltage system producing the correct voltage? Is the control voltage present at the gas valve when it fails to light? Call the U-FIX-IT store nearest you for personalized help in locating your problem.
Jim
I have same cycling problem but have no flame sensor. Schematic says i have a pilot sensor but the only thing going to it is the ignition wire. No lo v wire at all.
Bill,
Call the store nearest you for more personalized help. We will need to see your schematic to get you started in the right direction.
Jim
After cleaning flame sensor, checking ground, board connection, exhaust for blockage and none of that fixed it. We found the exhaust pipe hanger was poorly hung and was working its way out and this caused the exhaust to droop and hold condensation which would shut the unit off also. It seems to be fixed now.
Tammy,
Your experience is the reason I always fix whatever is not right, even when it seems unrelated to the problem I am trying to solve. The seemingly unrelated little thing has often proved to be the cause of the larger problem.
Good job finding the problem. Thank you for sharing.
Jim
my Gas furnace come on and lights, it stays lit until the main burner kicks on . as soon as the burner kicks on the flame goes out. I have cleaned the flame sensor and I have even checked the sensor appears to be working. could you please help.
thank you
David,
Your description is the classic symptom of a dirty flame sensor. If all your safeties check good and the control board ground is good. You may have a bad control board. Call the U-FIX-IT store nearest you for additional help.
Jim
Thanks for your info. Simple fix and very easy to do. Glad I found you and saved some money. Happy new year.
Dear Jim,
Thank you for your service to the USA! I truly appreciate the advice on this issue. I followed your directions and all is working fine now. Thank you again!
Jim Sachs
Thanks! 10 minutes of my time and it was fixed! Now to bill my landlord…. 😉 LOL
Woke up for Christmas only to find our house was 63 degrees on a 30 degree day. I knew there would not be much luck in getting a repair man out today without spending huge dollars. Found this article online and performed the repair.
The furnace has worked all day since with no issues.
Thanks for the great advice.
Brent,
Glad we could help.
Thank you for letting us know about your success.
Jim
I was laying in bed 30 minutes ago noticing that the burner was kicking off after about 5-10 seconds of burning, did a quick Google search, removed, sanded and reinstalled the flame sensor and all is well now. Oh thank goodness for the Internet!!!. It would have been a cold night and much $ and frustration to call the “furnace man” out tomorrow to do what I just fixed myself.
Dan H.
Lake Sherwood, MO
Dan,
Thank you for telling your story. It always fun to hear that we made someone’s day better.
A warm night’s sleep is a beautiful thing.
Jim
Hi Jim,
Let me begin by saying Thank You for your military service! We wouldn’t be an awesome country without selfless veterans like yourself.
I have a Luxaire gas furnace that is giving me fits! There are 4 burners that are to have a blue flame once the igniter heats the gas. Only 2 of mine light. The one directly behind the igniter and the one beside it. The other two will not light unless I clean the flame sensor which I have done a half dozen times in a week! Used sand paper and emory cloth. I’ve seen flame sensors before that doubled their size due the the build up and they still worked. This one has been shined like a mirror and will only allow one or 2 cycles before the furnace shuts down again! I’m at a loss to explain it! “They” say that sensors rarely go bad and cleaning is about all they need but where am I going wrong! Can you explain?
I have a Lennox furnace and my flame sensor is connected to a metal pipe and another piece it does not just come out like I seen in videos. I can take the screw out and clean the top of the sensor but like I said its attached to other pipes and things. Any suggestions would help. My furnace comes on for 1 minute then shuts down. I got a code blinking 7 times and its said its the flame sensor isn’t grounded. I don’t know what that means? Thank you.
helped
Nice try, but you’re wrong. The ignition board delivers 90-120V AC low amp through the flame sense rod. The flame rectifies the AC voltage to DC voltage ( the flame is a terrible conductor and only conducts half the sine wave changing the AC to DC voltage) and the ignition board is looking for 3-5 microamps through the ground.
John,
Thank you for the memorable explanation of the rectification process.
Jim
I too had the same symptoms and after spending over three hundred dollars on a new sensor, control board, and igniter all of which left me with the same problems. The culprit happened to be a Robin decided to build a nest in the chimney stack. Once I cleared it out the heater fired up and worked fine. Since i kept all the parts I replaced I won’t have to spend any money if one of those items should ever go bad.
Frank,
Thanks for the story. The Robin definitely set up a difficult to diagnose scenario. I admire your tenacity. You won!
Jim
OMG,, I’ve been having the same trouble after we just received 6 feet of snow! Kept trying to light it now that u mentioned the birds nest I realized it was the snow on the roof not allowing airflow, unfortunately before reading this flames shot out in my face and almost blew me through the wall. I’m lucky to be alive. Thank you all so much for your input on the internet to help others! GOD BLESS, & again, Thank you, Lisa
Lisa,
I am glad you are O.K. Good job of suspecting the snow was your problem.
Thank you for sharing your experience.
Jim
Had the same problem myself and this fixed it. Thank you, thank you!
re-posting, since you deleted and didnt respond to my first inquiry. somehow, I suspect this will be treated the same…
I have an 8 year old Lennox G43UF-60D-135-06, new home installation. 2800 sqft, 9.5′ ceiling in basement. unfinished. draws from internal room for combustion. Vent pipe from outside for fresh air not blocked. Gas furnace. for 7 years it was fine. Now it is exhibiting the dirty flame sensor – “clean me” every other month. The diagnostic code on the control circuit is always the same – Slow RED flashing LED mated with Fast GREEN Flashing LED (low flame sense detected). It started about a year ago, cleaned flame sensor – was fine for 3 months. Then became worse. (about once a month the error code would return). As of 3/30/15, the flame sensor has been cleaned three and replaced 4 times. Also replaced: Circuit board ($400) and gas control module ($600). 3 different repair businesses have looked at my furnace – yet the problem remains unresolved. About every other month, the flame sensor must be cleaned. (which isn’t an easy task on this model, given its mounting location within the furnace) – flames within the chamber appear normal, blue. air intake has strong suction. no blocked exhaust. filter is replaced on normal intervals. flame sensor isn’t loose and sits well adjusted in the flame. Michigan house. Thoughts, comments – much appreciated. thanks!
John,
I apologize for the delay. As you are painfully aware, your malfunction is going to be something out of the ordinary. You have been thorough in following all possibilities as they were posited to you. I do not have an easy solution for you. It appears from your narrative that cleaning the sensor corrects the problem when it occurs. This would lead me to look for something that changed in your house at the time your problem began. I would be especially suspicious of anything that changed the composition of the combustion air. Pool chemicals, cleaners, paint, scented candles etc…; anything that might produce fumes that could be sucked into the combustion air. I suspect the problem is external to the furnace and will appear to be totally unrelated. Something changed after 7 years of trouble free operation. The mystery to solve is – what changed? I wish I had something more definite for you. I hope this helps. Please let me know what the problem is when you find it. I would like to know.
Jim
I have a lennox furnace. The furnace burners kick on and run for a couple minutes and then shut down. The ignitor re-lights and the flame kicks on again. Then, the flame shuts down again after about a minute. The ignitor lights up again but, the burners will not re-light. If I shut the thermostat off for about 15 minutes, I can then get the burners to light up again but, the same thing happens where the burners turn off and on. I cleaned the flame sensor but, the same thing happens, again. Thanks.
I can tell you the cost…104 to clean and warranty it for a year. The last time I must have had a random acts of kindness vibe going bc the guy cleaned it for free as part of my maintenance contract. I have the same contract but a different serviceman and that is what he charged. But I’m a single woman and afraid of gas, so I’m just happy it’s working now.
I live in western Massachusetts and tonight the outside temperature is expected to drop to -14 degrees Fahrenheit (Yes, that’s negative or BELOW ZERO)! My gas furnace was cycling on and off repeatedly and so I turned to Internet searching and came across your site.
The fix in this post did the trick. I cleaned the sensor with fine sandpaper, put it back together and turned the unit back on. It fired up and has been running fine for over an hour.
Thanks very much!
Wow – That is brutal weather. I am happy we could help in time.
Had this problem, dusted my sensors with some brillo and tissue (to remove dust). Fixed! THANKS!
Glad we could help.
I have to clean my flame senser about once a month or my furnace stops working. I was told by a friend who does heating and plumbing that I most likely have a crack in my heat exchanger and my whole (14year old) furnace needs to be replaced. We have two co detectors in the basement by the furnace and more on our upper levels which have not detected any problems. Today I noticed that the exhaust pipe that runs from the water heater and connects to the exhaust pipe of the furnace before going up the chimney, is split open along the top side. I taped it closed for now with aluminum tape and my furnace seems to be running fine right now. Could this be my problem?
Heather,
Good job spotting the split in the pipe! I always recommend fixing the most obvious problem first. I have been pleasantly surprised how often this practice results in seemingly unrelated problems going away. Yes, this could be what is causing your problem. I recommend replacing the defective exhaust pipe and making certain there are no other leaks in the exhaust pipe.
Jim
Thank You for the info on this page cause it was very helpful my furnace is now heating
Thanks! Cleaned the fire rod with steel wool and the furnace is working great. Thanks to tips like these, Ive been able to repair all of my HVAC issues for a total of 30 dollars. I can only imagine what a HVAC company would charge!
Hi,
My furnace will just stop working. The thermostat if set at 69 and the inside temp is 60 and it shows the unit is heating. The only way to get heat is to turn off unit and restart it. I did replace the thermostat thinking that was bad. Unfortunately it has not stopped the problem.
I have been reading about this problem and they state I need to clean the flame sensor which has not been cleaned in a while. Do I have to take that out of the
furnace. Or could this problem be in a board?
Thanks, Maria
Hi, my suburban sf35 blower runs non stop. Limit switch was replaced after 2 weeks of bypassing (when furnace worked fine while waiting on part). Now the burner seems to fire for about a min but never seems to catch. It is warm in here but the blower constant running is driving me crazy.
Hi my name rene I am having problems with furnace it keeps short cycling ; I have replaced smartvalue ,control board,high temp limit, and rollout limit any help wuld be much appricated thanks
I have been having issues with my furnace. The flame lights them goes out right away. I have cleaned the flame sensor several times and it works for a few days and then I have to do it again. Am I not cleaning it good enough? Do I need to buy a new sensor? Could their be another issue? Let me know what you think and thanks for your help.
-Cory
Hey I have a Colman evcon gas furnace ive checked everything it had a bad high limit it was a 150 I changed it and now the flame and blower shuts off after 3 or 4 min and relights in a few min then blower restarts and repeats should I have a bigger high limit or what I appreciate your help
Will,
First, I recommend verifying that high limit you replaced is the component cutting the power to gas valve on your Coleman Evcon gas furnace. This can be done using a multi-meter.
Second, If the new high limit is cutting the power and it matches the temperature of the old high limit I do not recommend installing a higher temperature. The high limit only cuts out when something else is wrong. Look for something causing reduced air flow through the combustion chamber. A faulty induced draft motor, a restriction in the exhaust pipe or insufficient make up air are places to look.
A hole in the heat exchanger can also disturb the air flow patterns causing the high limit to cut out. This is an extremely dangerous condition as the gas furnace can leak carbon monoxide gas into the living area of the house and poison the occupants. If you suspect a hole shut down your furnace immediately until you can test it. Call your nearest U-FIX-IT Appliance Parts store if you have any more questions.
I have the same flame out problem as described above. Have cleaned the flame sensor, checked all the connections and made sure the ground wire was secure. The problem still persists. The flame stops as soon as the main blower comes on. The system then immediately recycles with the ignition cycle etc. A friend said maybe the controller board is bad and needs to be replaced. Wouldn’t there be other issues if this was the case. What else can i do to diagnose this problem?
I have the exact same issue. Everything works as it should until blower stars and burners stop immediately. 3 flashes code appears.
Did you sole this problem? Thank you!
I clean flame sensor it still short cycle could it be pressure switch
In most systems a pressure switch failure will prevent flame ignition. Here are three more items to check. (1)Check for a good connection between the flame sensor and the control. (2)Check the sensor is positioned in the flame. (3)Check the ground for the control board. The pathway for low voltage electricity runs from the control board to the sensor through the flame to the burner back to the control by way of the ground for the control board. If these three steps do not identify the problem, bring the wiring diagram for the furnace to U-FIX-IT and we will help you work out a plan for finding the problem.
I have a BRyant 80 series heater (HVAC)system. Mine has Puron for the AC.
I am very interested as to what this problem is cause I am having the same problem. I cleaned the sensor (maybe not good enough?) but the problem
persists. Cept now the flame will stay on for about 1 or 2 minutes, some
time maybe up to 5 minutes. but it will flame out quickly. Im considering taking thesensor out and cleaning it again. I now have some 4000 grit sandpaper,
maybe that will do the job? Will the folling answers be sent to me or do Need to
come back and check a couple of times daily?
The guys at my U-Fix-It had realy great guys to deal with. But this is Sunday, I think they close on Sundays…. Kirby
Yes, cleaning it again is a good idea. It does not take much effort to clean the sensor since the coating is very thin and easy to remove. In addition, you may want to inspect the condition of the ceramic insulator and the wire attached to the sensor for cracks or breaks. Check to make certain the electronic control board has a good ground. The circuit for the sensor is completed through the furnace frame and the ground for the board. Please call back to the store if you continue to have problems. You are right, U-FIX-IT is closed on Sundays to attend church and be with family. Thank you for your inquiry.