# Lennox Furnace burner won't stay on



## stevea7777

I've got an 88,000 BTU Lennox Elite Series Surelight natural gas furnace (model #G50UH-36B-090-02) that comes on when the thermostat calls for heat but the burner will only stay lit for about 3-4 seconds. It then tries to light again and does the same about 3 or 4 times and then it just quits until I lower the thermostat below the call for heat and then move it back up. Any ideas what would cause this? I'm in Montana and it's below zero right now. Thanks.
Steve


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## yuri

Clean the flame sensor AND the face of the burner it sits in front of with fine emery paper. The flame sensor sits directly in front of the left burner. Looks like a thin nail and is held in place with porcelain. Has a 1/4" screw holding it to the sheet metal. Do this once a year and you will be okay. Try keep the humidity down in the basement and put any fabric softener tissues in a ziploc airtight bag. The fumes from it corrode flame sensors. Saved ya $150.


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## dinotheminer

i have the same problem i tried this if it works i will buy you all the beer ya want. i am in saskaoon and its - 25


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## danfrain

Thanks for posting this picture,Yuri. 

I've spent hours looking for one. I've read several places that I might want to take it out and clean it, and had no clue what to look for. I'm going to take someone's advice and wait until the parts houses are open before I try to take it out. I'm thinking they're less likely to break when replacement is less inconvenient than the middle of the night.

Thanks again!


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## stevea7777

*Thanks Yuri- Great help. It works now.*

Thanks so much for the help. It's been 21 degrees here in Montana and that was a huge help. The sensor didn't even look dirty but cleaning it with steel wool made it work. I really appreciate your help.
Thanks! Have a wonderful year and God bless you! 
Steve


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## yuri

Glad to Help. Pay it forward and help someone else.

Cheers


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## Dream Maker

Great fix Yuri, thanks. 14'F here tonight!!!


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## jjc1111

*Yuri - Great Fix! Thanks!*

It's Christmas Eve, and my furnace was exhibiting the problems described in this thread. I cleaned the flame sensor as you instructed, and the furnace works perfectly.

Thank you for sharing your knowledge with homeowners. It would have been difficult to find an HVAC tech during the holiday. Thanks again.


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## JohnnyTacks

*Thanks!*

Thanks Yuri! It's supposed to be -10 tonight and my furnace was doing this. I cleaned the sensor and voila! My family and I are very thankful!


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## yuri

Good For You!:thumbsup:!


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## KCRutherford

*Thanks!!!!!!!!!!!*

Yuri your post saved the day. It is 9 degrees here in Kansas City tonight and my house was getting cold. I cleaned the flame sensor and the heat is flowing again. Thank you for sharing your wisdom.:thumbup:


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## Vernster

*Thanks*

My furnace was acting up on Monday night here in Regina and I managed to get it working. After seeing the LEDs flashing and indicating it was a low signal from the flame sensor, I tried to locate one. Mission impossible. No one will sell gas furnace parts to home-owners and it is impossible to find the parts manual for a Lennox G50UH(X) online. Thanks a lot Yuri, your answer seems to have helped a ton of people.


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## yuri

Glad to help, over 3000 views, must have helped quite a few folks:yes:


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## echo6romeo

*Need more suggestions please*

I have pretty much the same problem. My Lennox G43 unit will ignite the burners (Nice blue flame) but they stay on only for about 3 seconds and turn off. This ignition process continues for several hours before the unit goes to watch guard mode: failed to ignite...

I have already replaced the flame sensor and the hot surface igniter . I have reset the circuit breaker and changed the air filter.... Any other suggestions?


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## echo6romeo

yuri said:


> Glad to help, over 3000 views, must have helped quite a few folks:yes:


 
I need more suggestions please....


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## yuri

Clean the face of the burner that the sensor sits in front of with emory paper. It has to conduct the flame signal from the sensor which gets converted to DC current thru flame rectification. If that burner is even slightly corroded or rusty it weakens the signal. Make sure the ground wire from the burners on the right is tight and not corroded if it has one.


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## echo6romeo

yuri said:


> Clean the face of the burner that the sensor sits in front of with emory paper. It has to conduct the flame signal from the sensor which gets converted to DC current thru flame rectification. If that burner is even slightly corroded or rusty it weakens the signal. Make sure the ground wire from the burners on the right is tight and not corroded if it has one.


 
I cleaned it today using the emory paper. It was some what dirty but I cleaned it well. This is a 7 year old unit. I put everything back in order and STILL the same problem :wallbash:

I'm starting to think maybe the circuit board is bad or what??????


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## marchvac

There could be many problems. The pressure switch could be dropping out the gas valve could be dropping out and you should check your flame signal with a meter. Has anything been done to the venting inside or out. Is anything in the venting?


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## echo6romeo

marchvac said:


> There could be many problems. The pressure switch could be dropping out the gas valve could be dropping out and you should check your flame signal with a meter. Has anything been done to the venting inside or out. Is anything in the venting?


 
There have been no changes to the venting. As far as I know, I don't believe there is anything in the venting...


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## echo6romeo

echo6romeo said:


> There have been no changes to the venting. As far as I know, I don't believe there is anything in the venting...


 

The LEDs are both flashing fast= normal operation, but the burners keep going through the same cycle of only staying on for about 3 seconds... The fans are working fine. Its about 67 degrees inside, and I have the thermostat set to 80.


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## marchvac

before you condemn the board you should get a votmeter and check the pressure switch, gas valve and limits. It could also be the thermostat. You would have to disconect the wires from w and r and jump them out.


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## yuri

Why did you change the igniter? Where did you get it from? There are several versions of that igniter and they have to match the model AND series of that furnace and circuit board. Hope you got the right one. Universal igniters DO NOT work with Lennox units as Lennox uses a reduced voltage system for them. Marchvac is heading you in the right direction.


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## marchvac

Hey Yuri long time how is things. I had to do a job in regina last fall


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## echo6romeo

marchvac said:


> Hey Yuri long time how is things. I had to do a job in regina last fall


 
I purchased the igniter because I thought that may be the problem which wasn't. It's not the Lennox original part but looks pretty close. Either way, when I installed it, nothing changed and it still ignites the burners like the other one did. Now the flame sensor I purchased from Cozyparts and its the original part. I do have a volt meter (not digital) but not sure how to test the pressure switch, gas valve and limits...


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## echo6romeo

Here's my meter.


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## marchvac

After the furnace lights the pressure switch should stay closed which means you should have no voltage across the two terminals. If the gas valve has 24v and shuts off then the valve is bad. But it could also be the thermostat.


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## echo6romeo

echo6romeo said:


> Here's my meter.


I'm not even sure if this type of meter will work.


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## echo6romeo

marchvac said:


> After the furnace lights the pressure switch should stay closed which means you should have no voltage across the two terminals. If the gas valve has 24v and shuts off then the valve is bad. But it could also be the thermostat.


Can you please explain how to check the pressure switch with the volt meter (I'm new to the volt meter and the furnace). 

Also, just an update, the unit has been running for several hours now since I reset it to clean the burners. The fan is always running, the LEDs are displaying normal operation, the igniter is igniting the burners which will burn for 3 seconds before turning off. This process of trying to keep the burners on for more then 3 seconds keeps repeating after about every 20 seconds of rest/ burners off. Still no lock out mode.


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## echo6romeo

echo6romeo said:


> Can you please explain how to check the pressure switch with the volt meter (I'm new to the volt meter and the furnace).
> 
> Also, just an update, the unit has been running for several hours now since I reset it to clean the burners. The fan is always running, the LEDs are displaying normal operation, the igniter is igniting the burners which will burn for 3 seconds before turning off. This process of trying to keep the burners on for more then 3 seconds keeps repeating after about every 20 seconds of rest/ burners off. Still no lock out mode.


This is a picture of my unit: 
Lennox G43UF -36b-070-08


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## echo6romeo

echo6romeo said:


> This is a picture of my unit:
> Lennox G43UF -36b-070-08


It's FIXED!! I just unplugged the red tube from the pressure switch and blew in it to clear it. That's it.


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## college A/C

*message to echo*

You still need help echo?
Have you tried to clean the draft blower tube yet? The rubber tube is connected to the induced draft blower housing where the flue pipe comes into the unit. remove that end of the rubber tube abd clean out the fitting with a small piece of thermostat wire or stick pin (what ever will fit through the hole aboutv 1-2 inches) if you meet resistance then scale has partialy plugged the hole and the wire should clear it. put the tube back on and try to relight the furnace. let me know if it works for you


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## echo6romeo

college A/C said:



> You still need help echo?
> Have you tried to clean the draft blower tube yet? The rubber tube is connected to the induced draft blower housing where the flue pipe comes into the unit. remove that end of the rubber tube abd clean out the fitting with a small piece of thermostat wire or stick pin (what ever will fit through the hole aboutv 1-2 inches) if you meet resistance then scale has partialy plugged the hole and the wire should clear it. put the tube back on and try to relight the furnace. let me know if it works for you


No thank you. I fixed it by blowing into the red tube which is connected to the pressure switch. All fixed.


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## pmorganii

*Fixed*

Things are getting cooler in Texas and my downstairs furnace would not light...upstairs worked fine. After searching the internet, I found this forum. I removed the ignitor sensor and cleaned...no luck. Then I found this answer...removed red rubber tube, cleared it and cleared both ends out with a long paper clip...fired up after that. I could hear some tinking when the fan turned back on...assume that was the blockage. 

Thanks!! Downstairs is warm again.



college A/C said:


> You still need help echo?
> Have you tried to clean the draft blower tube yet? The rubber tube is connected to the induced draft blower housing where the flue pipe comes into the unit. remove that end of the rubber tube abd clean out the fitting with a small piece of thermostat wire or stick pin (what ever will fit through the hole aboutv 1-2 inches) if you meet resistance then scale has partialy plugged the hole and the wire should clear it. put the tube back on and try to relight the furnace. let me know if it works for you


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## TwitterD

Hi All,
I had the same issue happen to me last night and this thread helped me resolve it. I had a hard time figuring which red tube to clean out on my unit "Lennox G51MP", so I wanted to include a picture in case it helps someone else in the future.
Thanks.:thumbup:


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## jayfern10

Thanks for posting the Fix Yuri.

I had similar symptoms and cleaning the flame sensor worked. For a while I thought it might be the blower because it would come on at times, but only run for 3-5 minutes. I'm glad I found your post. Temperature in NY tonight about 29 degrees.


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## JohnPlo

*Similarly connected?*

Dear Santa :santa:, 

I know you won't be back until next year but I am left out with a sick furnace. Please help me. 

I have a Lennox Elite Series Furnace with pretty much connected problems with the ongoing thread. 

This one, my furnace/burner kicks in but the blower would not come on. Actually, the blower clicks in, on and off several times and if I get lucky sometimes it picks up, otherwise, it just clicks and clicks. It's trying to turn on the blower but somehow it just can't. 
Because of the many clicks, I have to manually turn off the Thermostat. At this point the burner's heat is wasted bcuz it was never circulated and picked up by the blower. 

Any input would be greatly appreciated... grrrr (I'm cold).
Thank you


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## carmon

sounds like the board is having issues......:furious:


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## JohnPlo

Carmon, thanks for the input. 
Any specific I need to do on the board? I don't feel like replacing the whole board. But if I do, what are the possibilities to check first before it goes into the dump? I am the type of guy that gives a little fight first before I fold. 

Thanks.


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## Dave Sal

Does your furnace have a little window over the integrated control panel? If so, take a peek in there and see if there are trouble codes flashing on the LED's. I have a Lennox G60 which has this diagnostic feature.


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## JohnPlo

*Blower not kicking in after burner stops*

Hi Dave, unfortunately, I don't have the screen window. I have a Lennox Elite Series G24M4 model which is fairly not that old I think. 
But I notice there are 2 blinking LED lights. I'm not really sure if that's normal (is it?) since I didn't pay attention to it the first time if they're supposed to be stationary in the normal state. 

What I did though is cleaned up the thermocouple/flame sensor and it's still remained to be seen at this point. I did this bcuz the burner does not hold long enough - it turns on for about a minute then off in seconds and back again. 
I thought this probably confuses the blower when to do her own time dance... 

Thanks again


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## Dave Sal

JohnPlo said:


> Hi Dave, unfortunately, I don't have the screen window. I have a Lennox Elite Series G24M4 model which is fairly not that old I think.
> But I notice there are 2 blinking LED lights. I'm not really sure if that's normal (is it?) since I didn't pay attention to it the first time if they're supposed to be stationary in the normal state.
> 
> What I did though is cleaned up the thermocouple/flame sensor and it's still remained to be seen at this point. I did this bcuz the burner does not hold long enough - it turns on for about a minute then off in seconds and back again.
> I thought this probably confuses the blower when to do her own time dance...
> 
> Thanks again


The two blinking lights are your diagnostic lights. All you need to do is see how they are blinking and make a notation. You can usually find the trouble codes on a sticker on the front of your furnace. The LED's can blink alternating fast, alternating slow, both on fast, both on slow, etc. I've attached a pic of the trouble codes sticker on the front of my furnace. The little viewing port is right below.


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## JohnPlo

Thanks for the info... I may search the code online since I could not decipher the characters. It's a little too small for my eyes. 
I could see that your unit is pretty new. 
My unit is the older type. When you take off the metal mail cover, it will expose the bare circuit board which does not have any troubleshooting codes. I guess, since it's the same maker(Lennox), I would assume they have similar hints of error. 
For now my unit is still under observation since I cleaned the flame sensor with a fine steel wood sponge. 
Also, I replaced the original vintage thermostat (thinking it was the problem) with a Venstar T5800. It's really cool and has a wi-fi where you can remotely control the temperature using your Android cell or iPhone apps. Just ordered the wi-fi unit at Amazon. I just got lucky and bought it at Craiglist for 60 instead of the full price of 230! and it's new too. 
It's feels like the 'Back to the future' stuff. From a 1900 Thermostat to a 'Star Trek' era. Check it out guys. I'm sure it's worth it when you sell your house eventually.


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## ChefJamesE

I will be trying this tomorrow. Hope it works. Freakin' cold tonight. Been sitting around 6-9 deg. F overnight, up to around 20 during the day for the last week or so.


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## JohnPlo

To all: Thanks for all your input.

I took apart my Lennox Burner (it's pretty simple thing: just need a little muscle to unscreaw a few bolts-5 I think), separated the burners (clean them up), then I took out the Flame sensor which is actually the Thermocouple. 
I used the finest wool sponge (used for wood) and wiped the rod gently a few times. Put it back altogether.
So far, it worked. Never had any problems since(well, cross my fingers). It's been over a week now (using it 3-4 times daily). 

I guess it's the beginning of a new year. 

Thanks again guys... :thumbup: Happy New Year!


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## texborn1

*It Works*

Thanks for your info Our furnace would not stay lit. Cleaned flame sensor but did not help. Called for service man. He checked voltage from flame sensor and was OK. Said it must be circuit board and would cost $470. We first said OK but on checking internet found replacement board of about $200. I put in the board but still had the problem. On reading this post I thankfully read until I saw about the red hose. Tried that and SUCCESS. I will keep the old circuit board since we have two of the same furnaces and will therefore have a backup.
Thanks greatly for everybody sharing their problems and help.
:thumbup:


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## vbeltran

*My Furnace Lennox G51MP is not staying ON*

I own a lennox G51MP and is nor working properly, I Called a company to see what was wrong and they said that is the Control board or the surface ignitor.
Whe I turn on the furnace, the furnace works and it reaches the temperature that I have set on my Thermostat but when it calls again for heat the induction motor stays on and it does not shut off and it gets very hot as well.
I would like to know if soembody has an Idea on what's wrong with this furnace the comoany that I called wants $1,300. to fix my furnace this is alot of money this furnace is only 7 years old, I know that is something simpler that what they are talking about also the person that came to see the furnace told that it could be the pressure switch this furnace has two.
Hopefully somebody will answer my queistion and concerns


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## vbeltran

I own a lennox G51MP and is nor working properly, I Called a company to see what was wrong and they said that is the Control board or the surface ignitor.
Whe I turn on the furnace, the furnace works and it reaches the temperature that I have set on my Thermostat but when it calls again for heat the induction motor stays on and it does not shut off and it gets very hot as well.
I would like to know if soembody has an Idea on what's wrong with this furnace the comoany that I called wants $1,300. to fix my furnace this is alot of money this furnace is only 7 years old, I know that is something simpler that what they are talking about also the person that came to see the furnace told that it could be the pressure switch this furnace has two.
Hopefully somebody will answer my queistion and concerns


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## yuri

Is it the induction motor that won't shut off and gets hot. If so the the relay on the control board is sticking on and you need a new board. It is easy to change and you can get it from cozyparts.com.


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## acowlaughing

*The Red Tube!!*

Just wanted to say thank you for this thread.. Spent a couple days troubleshooting my issue of the burners going on and off. I cleaned the soot off the sensor rod, checked ventilation blocks, but cleaning the pressure switch tubes worked like a charm. There was a lot of build up in there! Was on my way to hiring someone to come out! Thanks again


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## yuri

Yur Welcome:yes:


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## cmccluer

*Thanks, Yuri*

Thanks, Yuri. I cleaned the flame sensor and now we have heat! You are still helping people 7 years after your original post!
Kansas City, 2/5/15, 4 degrees.


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## yuri

It's an Oldie but a Goodie


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## hewley marks

*furnace did not stay on*

the information received did work by cleaning the furnace flame sensor:smile


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## Dave Sal

Had the same issue as the original poster a few days ago and found this thread. Furnace is back running once again. Thank you Yuri for sharing your knowledge and saving us DIY'ers from the cold.


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## yuri

Glad 2 help.:wink2:


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## TryToBeHandy

Need To revive this Thread, have a Lennox G43UF furnace that going crazy.

A week ago the furnace started to act up, it would take 3 tries to start until it stopped working with the green LED on and the Red, after looking further found the inducer gone, replaced the inducer and the furnace worked for 2 days and it started to act up again as follows:-
First try:-
1 - Water into humidifier
2 - Inducer fan on
Stops

Second Try
1 - Water into humidifier
2 - Inducer fan on
3 - Ignition start
4 - Flames on for 1 sec
Stops

Third try it works and as it works it give error code about flame sensor for 1 sec, then normal operations and shuts down after 5-6 minutes.

Today I replaced the flame sensor and it is still doing tricks as follows:-

First try:-
1 - Water into humidifier
2 - Inducer fan on
Stops

Second Try
1 - Water into humidifier
2 - Inducer fan on
3 - Ignition start
4 - Flames on for 1 sec
Stops

Third Try stops and error code Green Light on and Red Off complaining about pressure switch after a while the lights go back to normal and then it make the same sequence but it actually works for extended period now 15 minutes or more and then quits with just the green light one and Red off and so on and son forth.

Please help


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## yuri

Is it giving you a pressure switch fault code? If so check your vent pipes outside for obstructions.

Where did you buy the inducer. e-bay?

Was it in a sealed box? Lots of e-bay stuff may be used. If it is not turning fast enough or a OEM Lennox unit it may not be producing enough draft to close the pressure switch. That should be checked with a manometer.

cozyparts.com for Lennox parts.


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## TryToBeHandy

I am in Canada, and I got it from an HVAC supply store, it is of the same manufacturer as the original "FASCO" and I double checked the part number from multiple sources.

As for the vents I checked as much as I can and they seem to be clear, as when I was replacing the inducer I can feel the cold air draft coming through the vents.


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## yuri

Take the hose off the pressure switch and poke the hole thru where it attaches to the plastic collector box on the furnace with a nail or small drill bit.

If it still acts up then you need a Pro to check the vacuum on that port with a manometer and compare it to what the switch is rated for. Or buy yer own manometer.


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## TryToBeHandy

I already took the red hose and run air through it and it is not blocked should I do the same with the black hose as well


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## yuri

Do both of them. It is not the hose that is the problem, it is the port they attach to on the furnace NOT the switch. It can get partly plugged with scale.


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## TryToBeHandy

Okay, cleaned both tubes, installed a new Thermostat, the furnace worked perfectly for 2 days and today it started to do the same thing, takes 3 tries to come on.:vs_mad::vs_mad::vs_mad:


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## yuri

What EXACTLY is it doing?

1) Does it start and the exhaust fan keep running. Yes or No.

If it does and then you get a pressure switch code then you need to check the draft on the switch with a dual port manometer to see if it is enough.

2) If the exhaust fan shuts off prematurely then your circuit board is probably faulty. The relay for the exhaust fan is probably weak and failing.


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## TryToBeHandy

First Try:-
exhaust fan is on for 1-2 seconds and then shuts off - no error code

Second Try:-
exhaust fan is on , ignition surface is on, flame is on for 1 sec and then shuts off - no error code

Third Try
it works

There are no error codes at all now, it just always take 3 tries to come on.

Strange thing is that every time I Shut it off to replace something it just works for a day or 2 no problems and then it is back at it.


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## yuri

Remove the thermostat wires on the circuit board after labelling them and taking a picture of them.

Jumper R to W and then press the door switch in and it will start. If the exhaust fan does not stay on then the circuit board is faulty.

I have a G51 and after 10 years the relay for the exhaust fan got weak and it would cut in/out for no reason. I replaced the board.


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## muswell65

Good threads never die...

New to forum, and stumbled upon this thread AFTER my problem was resolved. So I'll share my experience...

In this case, my Lennox G50UH proceeded through all normal start-up states until the end of the 45 second delay before blower motor starting. Burners were running the entire time until the blower motor started - then the gas would shut off and the cycle would repeat. After a couple cycles, it would stop starting for some time - then repeat. DS1 LED was off; DS2 was slow blink, which suggested it could be the pressure switch - but this didn't seem right, since the inducer start-up delay proceeded normally, indicating the pressure switch was first open, then closed after the inducer started.

I was about to give up and just replace the control board, but on a whim, pulled the tube off the pressure switch and cleared both the inducer motor and pressure switch ports. Problem solved!


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