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Unregistered
December 16th, 2004, 07:54 PM
I inherited a Lennox G12 gas furnace that will light up ok with the electric igniter and burn for a short time, approx 1 minute, then shut down. Any ideas what can be the problem? Thanks

mdshunk
December 17th, 2004, 02:55 PM
Look in the little "window" that is in the lower section of the furnace (where the control board is at). When the unit shuts down, there should be a flashing light that is flashing a "code". See how many times that light flashes, and then remove the lower sheet metal panel covering that board. On the inside will be a sticker that says what that code means. Repost with the flash code, and what the sticker says the code relates to, and we can help you from there.

Unregistered
December 17th, 2004, 05:02 PM
Thanks MdShunk for a fast response. I looked at the unit and could not find any flashing light. This appears to be an old unit. The model number on the decal is G12Q5E-137-6. The SN is 5880K 03060. It appears that the Penn Pilot Ignition that came on the unit originally was replaced in 1990 with a Johnson Controls Series G60 Electronic Pilot Spark Ignition Control. Is the light you have asked me to check the code on the G60 or is it in the wiring make up box located in the lower section where the blower and hammock filter is? I frankly think this unit is so old it doesn't have a "circuit board" in it. But, I'll keep looking.

mdshunk
December 17th, 2004, 05:33 PM
Okay, sorry about that. Yes, the Johnson G60 module is pretty old by now in and of itself. It has been supersceded twice by a newer style module. I think that you need to clean the flame sensor. It is a little rod that sticks in the flame connected by one wire to the G60. Remove the sensor and clean it with a little steel wool or fine emory cloth. Replace it and the unit should fire well for you. If not, repost and we can troubleshoot a little further. The flame sensor might be a little tough to get physically out. If you can reach it to clean it while it's still installed, that would be okay too. I've used one of those little brass bristled toothbrushes for cleaning these before. This is a pretty easy furnace to work on, but a little tough to troubleshoot by message board posts. Below is the sequence of operations for your G60 module, and I've highlighted where your unit is shutting down.

Unregistered
December 19th, 2004, 07:23 AM
The flame sensor, pilot flame holder, and the spark ignitor all seem to be mounted as a unit. I haven't tried to remove the unit (looks like a single screw holds it in place). I bought one of the brass bristle brushes at Lowe's, but it is too large to get between the spark igniter and the flame sensor. Can I get in any trouble removing the single unit and then cleaning the sensor? I would hate to not have any heat this week of Christmas. Do I disconect the pilot gas line? It looks like I would need to so that I can remove the unit. How critical is the placement of the unit when I reinstall? It appears it will go back the same way it came out since there is only one screw holding it in place. Any ideas where I can purchase a "small" toothbrush or smaller brass bristle brush? The flame sensor appears clean to the naked eye. Thanks for your help.

mdshunk
December 19th, 2004, 08:07 AM
Yeah, the flame sensor is sometimes hard to get to without removing the pilot assembly. Depending on how your furnace is laid out, you may or may not have to unscrew the aluminium pilot gas line. You can disconnect it at either end (gas valve or pilot burner). Use a little flare nut wrench or Crescent wrench to undo the nut. Even if the flame sensor looks visually clean, it's generally a weird oxide layer that builds up on the sensor that prevents proper flame sense rectification. Also, the "ground" at the pilot burner is very important for proper flame sensing, since this completes the circuit for the flame sensor (one wire on the sensor, and ther return path is through the chassis). When you remove the pilot burner assembly, clean the rust off the mating surface of the pilot burner assembly and the mounting spot at the main burner.

Unregistered
December 21st, 2004, 07:35 AM
Thanks MDSHUNK for your expertise. I removed the pilot assembly, cleaned the flame sensor with a brass bristle brush, and replaced everything and it is working. At least its working now. This shut down was intermittent so I don't want to count chickens yet. I am really impressed with the clear instructions and promptness you responded to my distress call. Thanks again.

Unregistered
December 22nd, 2004, 02:37 PM
mdshunk, I googled over here and have EXACTLY the same furnace (G60 and all) with exactly the same problem. You have now helped 2 people out....thank you!

Unregistered
January 12th, 2005, 07:15 AM
MDSHUNK - I am having a similar problem with a very similar model (G12Q4E). Unit runs for 5 mins, shuts off for 20 seconds, then runs for 5 mins again, etc. I had my heating contractor (as I am not an expert) take a look and they determined the flame sensor is no good. But apparently Lennox does not make the same replacement flame sensor for this model any longer (house was built in '87). So they suggested a new package consisting of pilot assembly, flame sensor, gas valve, etc. Other than that and a little rust on the heat exchange, the furnace still is working fine. Does not appear to be any evidence of any cracks in the heat exchange. From the other postings, it appears the flame sensor can be a common problem in the older Lennox gas furnaces. Is it worth trying to fix or should a new model be considered?

Thanks!!

mdshunk
January 12th, 2005, 07:05 PM
I can't speak about something that I can't see, but I can say this- I have only ever seen one bad flame sensor in my entire lifetime. They don't "go bad". The only bad one that I saw had the porcelin insulator cracked from the abuse of the monkey who tried to "fix it". They are just a heavy steel alloy wire that passes through an insulator with a wire hooked to it. They are dirt simple. My gut tells me that you might be getting snowed, but I can't be 100% sure about that. In any event, that flame sensor assembly was originally made by Honeywell, and should still be available through them.

Unregistered
February 2nd, 2005, 12:22 PM
mdshunk, I too googled my way here. Thank you so much for the info, I had/have the exact same problem with my lennox G2603-50-1. I followed your advice, and it seemed to work at first, but now having same problem again. It seems to stay lit some of the times, and other times the flame will go out before the thermostat tells it to. It kept going out last night, then seemed to run fine this morning, and now the flame keeps going out after a few minutes, any other ideas? I actually cleaned the sensor twice and all the metal contacts that ground it to the chasis.

Unregistered
February 18th, 2005, 06:09 AM
g12q4e - flame comes on for 5 minutes, then goes out, refires, then out. this cycle will continue. unit is clicking (inginition noise?). cleaned flame sensor as you suggested. worked for a day or two but back to same problem. Any other ideas??? back in early Jan, my blower motor would turn off after running for about an hour or so (and was not sounding too healthy) had my hvac guy come out they wanted to replace motor with another but not an original part so they would remove and get belly-band. but I would be without heat for weekend - unacceptable. tech lube'd motor and it worked since (although with a little whinning). Ordered new lennox motor/cap but could not install (shoulder injury that week). should the pwr control module be at fault? - thnks

bmwpower
February 18th, 2005, 06:37 AM
Possible condensate line clog?

Pugman
February 18th, 2005, 06:44 AM
I have a Lennox furnace I had installed in 2001 and every 3 months almost like clock work I have to clean the flame sensor to make the furnace operate properly. Is this normal maintenance with these things or do I have a bigger problem.

thomasm
February 22nd, 2005, 09:07 PM
flame rods fail fairly often due to moisture. The connection between the wire and the rod is often crimped with a regular steel sleeve that will rust. the corrosin will cause the steel to expand and cause a crack in the insulator. In severe cases the insulator will fall into pieces. The most common problem with lockout during the heating cycle is insufficient pilot. The pilot burner should always be cleaned if the flame current is low. JC accepts asa low as .2 ľA in many controls.

DesparateMan
February 26th, 2006, 08:51 PM
I also have Lennox furnace that the flame wouldn't turn on or turn on for less than a minute. I've been reading this thread and just can't picture all the words and things you all have mention.</p>
I have fast flashing green light : Normal Operation - signaled when heating demand initiated at thermostat. Can someone point out where exactly where I need to check/clean before I get help and pay lots of money? I uploaded picture of my furnace open.

Philabob
February 15th, 2007, 08:14 AM
Thanks for the advice - I had the problem too, had to turn the heat off & on (reset it) until it would finally run then it would shut off & never turn back on. I cleaned the sensor and it has been running perfectly! Now - is the inducer motor supposed to run all the time?

sloooo
February 15th, 2007, 10:41 AM
No, the ventor fan will turn on 30 to 60 seconds prior to the burner turning on, and it will run for 30 to 60 seconds after the burner turns off. Whey you say all the time, do you mean just during the heating cycle, or even when there is no call for heat?

Philabob
February 16th, 2007, 03:15 PM
it runs constantly - even when it doesn't call for heat - it never shuts off

sloooo
February 16th, 2007, 03:45 PM
I also have Lennox furnace that the flame wouldn't turn on or turn on for less than a minute. I've been reading this thread and just can't picture all the words and things you all have mention.</p>
I have fast flashing green light : Normal Operation - signaled when heating demand initiated at thermostat. Can someone point out where exactly where I need to check/clean before I get help and pay lots of money? I uploaded picture of my furnace open.

You need to check your burner sensors. You should have a high limit sensor, and a pressure sensor. If you take a better picture of the upper portion of your heater, I'll try to point them out to you.

sloooo
February 16th, 2007, 03:47 PM
it runs constantly - even when it doesn't call for heat - it never shuts off

Your relay for the ventor fan is stuck closed. If I remember right, it's controlled by the main board. Sooner or later your going to have to replace your board, or you'll end up replacing the fan. It's not rated for 100% duty cycle.

Philabob
February 19th, 2007, 05:43 AM
Thanks I appreciate it. Can a person buy a main board or is it time for an HVAC guy to come in?

sloooo
February 19th, 2007, 06:49 PM
You should be able to buy a main board. Look up United Refrigeration on the internet and go to branch locator. There should be a branch within 30 miles from you. Make sure you pull the old board off (noting where each wire goes), and take it to them. They should have one instock. If not, they can usually get it next day.

amokay
October 26th, 2007, 06:35 PM
I have a lennox furnace model G12Q3E that wont light. It has been off for the last 5 months so I decided to get it ready for winter. I brushed off corrosion and vacuumed the burner, oiled blower motor, vacuumed blower area, and changed filter material on hammoc basket. After cleaning I turned up temp on thermostat and the igniter has strong consistent spark but pilot wont light. I removed the pilot tube from the gas valve and blew through it with compressed air but still wont light. I tried mant times to get it to start and one time I noticed a very small blue flame at the pilot tube but it was too small to heat flame sensor and went out quickly. I suspect that the solenoid in the gas valve is not opening fully because if if was a sensor that was bad there wouldn't be any gas released. I guess the pilot burner could be dirty but I thought that blowing air through it should clean it. Another observation is that last winter I noticed that the burner behaved oddly. Sometimes the igniter would spark when thermostat called for heat and the pilot would light then the main burner, but within a second the burner would go out and then relight less than a second later. Please comment with any recommendations.

sloooo
October 27th, 2007, 12:51 AM
Your pilot valve seems to be working since you can see a small blue flame. It's not enough to let the flame sensor see it so because of that, it might not be lighting the main burner. You need to check a couple of things. First, close your gas valve to the heater. Unscrew the flex going into the heater. Next, make sure your not smoking and open the valve for a second or two. Make sure you have gas pressure going into the unit. If you do, pull the pipe off going into the gas valve. You should have a screen on the inlet of the valve body. Check to make sure you do not have a restriction going into the valve. If you don't, replace the lines with gas tape and grab your meter.

You'll need to look at the schematic of your unit. It's usually behind one of the heater doors. You'll have 3 wires going to your gas valve. Valves have different stages, but your going to be looking for your pilot valve terminal and common. When you hear the sparker, you should get 24 to 30 volts at the pilot valve. This means your system is calling for the unit to ignite the pilot, not the main burner yet. If you still have a small flame with no restriction, you'll need to have the gas company come out and verify pressure to your system. Most heaters work on 3 to 5 inches of water. If it's above or below that, you'll have a non-operational or un-stable system. Either the gas company or a GOOD ac company can check this for you. The gas company is free, but the ac comapny will charge a service call. Let me know what you find out.

fordrules
October 27th, 2007, 09:24 AM
G12, is that a Lennox pulse furnace?
If it is, you may have other issues

amokay
October 28th, 2007, 07:30 AM
Thanks for the feedback. I have confirmed that I have 29V at the terminals for the pilot valve. I have also removed the pilot tube from the gas valve, and can hear and smell the gas coming out of the gas valve. I tried to blow air through the pilot tube through the flame spreader for the pilot light, but I don't think the air is getting through. I removed the flame spreader assembly but do not have anything small enough to push through the brass fitting for the pilot light. Since the unit is 22 years old and seeing oxidation on the exterior of the brass fitting for the pilot, I suspect that the tiny pin hole for the pilot is blocked. I will see if I can buy a new flame spreader unit from a local heating contractor tomorrow. If you have any ideas for how to clean or verify that I can pass gas through the pilot/flame spreader let me know.
THANKS for all the comments
Al

amokay
October 29th, 2007, 06:01 PM
Here's an update on my progress. I could not find any local dealer that would sell me the pilot burner and the house is chilly, between 45 and 55, so I yielded to call local contractor who said they would have furnace running today. They came, told me that I need a pilot bruner, it would take them a couple of days to get one (if they could because my furnace is 22 years old) charged me $100 for service call, and said it will cost another 325 to install.
I ordered the part on line for $59 and will have it by Friday. I understand that the contractor need to make a living and I am willing to pay for their service, but $325 for a $60 part and half hour labor is too steep.

sloooo
October 29th, 2007, 07:04 PM
You did pay for there service. You paid $100 for them to come and troubleshoot it. If your mechanically inclined, by all means call someone out, find out the problem, then repair it yourself.

DavidK
November 11th, 2007, 10:27 AM
I can't speak about something that I can't see, but I can say this- I have only ever seen one bad flame sensor in my entire lifetime. They don't "go bad". The only bad one that I saw had the porcelin insulator cracked from the abuse of the monkey who tried to "fix it". They are just a heavy steel alloy wire that passes through an insulator with a wire hooked to it. They are dirt simple. My gut tells me that you might be getting snowed, but I can't be 100% sure about that. In any event, that flame sensor assembly was originally made by Honeywell, and should still be available through them.

I have a Lenox G12Q5137-2 I can easily light the pilot, and it runs for a bout 1 minute and then shuts everything down including the pilot. When it's on the flames looks nice and blue and comes on strong but do not know why it shuts itself off. I really appreciated any help I can get.

ADSFire
November 21st, 2007, 12:48 PM
I really wished I could have read this forum last night. My elderly parents called last night and said the furnace was blowing cold air and a HVAC guy was on the way. Outside temperature was 17 degrees. The furnace was doing the cycling as well. Long ignition, pilot lighting, then shutting down while the blower continued to run. They guy checked the voltage, it was 25. The Johnson control module has a solid green LED which the guy said just meant their was power to it and did not tell whther or not there was a problem. He said the Lennox furnace was 10 years old and the original parts wear out after about 10 years. His explanation of why the furnace wasn't working properly was consistent with this forum. Then he said we needed to replace the pilot assembly and the Johnson control module because its best to replace them at the same time. No mention of just cleaning the flame sensor, which looked fine (i.e. no cracked insulator and minor black layer on the sensor. I kept all the parts just in case. After hours cost for ~1.5 hours of work and parts: $610.33. Hosed or not hosed? I couldn't let my parents freeze for the night, so I had to let him do it.

sloooo
November 21st, 2007, 04:53 PM
I think that's a far price. After hours cost and parts are the majority of that cost. If you were to have it done on regular hours, it still would have cost you about $500.

whughw
November 25th, 2007, 07:27 PM
ATTN mdshunk
Sorry to but in this thread byt I have a problem you may be able to help with
I have a g12 lEZNNOX THAT HAD A BAD FAN/LIMIT SWITCH....WAS NOT SHUTTING FAN OFF AFTER heat was off....Anyway after fiknding locally
that the part was not available...I got a replacement from Cozy parts
and it was drop shipped from Lennox....Installed the f&L control and it works as follows:
When fired up, after some time waiting for fan to come on...it shuts off instead ...after 30 seconds, fan will come on...then in 10 or 15 seconds
unit fires up again....does ok with the rest of the cycle I've tried to adjust
the temp on ;the f & L part...but doesn't seem to make any difference
if it is 70 or 110...it still works ther same way...
Do I have a defective new part? Will appreciate your thoughts
Hugh (whughw)

Boober
November 26th, 2007, 05:37 PM
Hello

I have a g12q3e-110-8 and have a similar problem. The unit will light, run for a few minutes then not try to light again. The fan runs constantly at a low speed and does not speed up. If I turn the unit off and back on it will light right up and run for a few minutes. The flame sensor is clean and appears to be working properly.

I believe the problem relates to the constant low speed fan. I am handy with a volt meter. Are there anthing I could test to verify the problem.

Thanks

ADSFire
November 27th, 2007, 06:38 PM
I think that's a far price. After hours cost and parts are the majority of that cost. If you were to have it done on regular hours, it still would have cost you about $500.

Well I guess its nice to know that is "fair" price for what was done. I was just questioning the need for flame sensor replacement vs cleaning. Is it true that Johnson Control Modules have the need for replacement after 10 years? He installed a Honeywell as replacement. Either way, it was an expensive education. I, of course, had to watch and learn. See one, do one, teach one. Thanks, slooooo.

sloooo
November 28th, 2007, 06:48 AM
As far as cleaning the sensor, you'll get a little more life out of it, but it will still need to be changed. Once they get cleaned, the life is coming to an end, so he was right to replace it. As far as your Johnson Controls that needed to be replaced, there really isn't a time that they go out. Could be 3 years, could be 20 years. Lot's of factors to the life span. He might see an average of 10 years in call's he went on, so that's the info he gave you.

MR Chase
November 29th, 2007, 06:04 PM
I just bought a house and the heater doesnt work well.

I turn on the thermostat, it runs for about 5 minutes and the shuts off. The blower doesnt come on either, what could the problem be? this furnace is only 4 yrs old

mattk_r
December 5th, 2008, 08:45 PM
I've had similar issues, they system would come up, combustion fan and blower would seem to work fine, burner lights and stays lit. After about 4 minutes or so it would shut down, LED 1 was flashing and LED 2 would stay lit.

I used the following guide I found on the net a while back: Lennox TS Guide (http://www.mattkyle.com/Files/furnace_ts.pdf)

No promises it'll stay up forever, but it'll be there for a while. Grab a copy and share.

While the symptoms of how it acted wasn't exactly the same as the guide described (other than LED 1 flashing/LED 2 on), the fix was as the guide said for the LED symptoms - the limit switches.

My model is an 80MGF, but this guide covers multiple models.

Another problem I had in the recent past was the switch that is activated off the diafram that checks the combustion chamber blower became plugged at the orifice in the blower casting (barbed hose connection coming out of it). Apparently the exhaust pipe has corroded over the years and the debris has formed a blockage. A small finishing nail knocked out the blockage and all is well.

hardy
December 19th, 2008, 11:58 AM
cleaning the gas sensor worked for me. thanks add one more to the helped column

hardy
December 19th, 2008, 12:00 PM
mdshunk, I googled over here and have EXACTLY the same furnace (G60 and all) with exactly the same problem. You have now helped 2 people out....thank you!

helped me to..thanks

kaisp
January 11th, 2009, 01:20 PM
I don't have this furnace, but my family woke up this 20-degree Sunday morning to frozen house because the one I do have exhibited the exact same behavior. I was sure I'd be spending the day waiting for a service guy who would charge me several times the normal rate for an emergency service call. But I found this post, identified what looked like a sensor (mine looks like a metal lollipop stick), did the work in no time and had a wark house and were off to church. Even though the sensor didn't look all that dirty, there must have been some kind of residue or something on there because the steel wool did the trick.

Thank you, Mdshunk!:adore:

P.S. Tip: I used a fine steel wool at first, the kind I use for smoothing out wood finishes, but it didn't make any difference. The rough stuff shined up the sensor a bit, and that's all we needed to have it do the trick.

chuckh
March 6th, 2010, 08:14 PM
Lennox gas furnace model #G12q3e-110-6 at my 88 year old mothers house probably 25 years old, with penn pilot ignition, would light for a second or two, then go out, repeatedly trying to light up. After several tries main burner would light and heat house. replaced heat sensor, no difference. Replacement ignition control box no longer manufactured. Updated control box out of stock for at least 3 weeks, cost $500 plus installation. Only option new furnace, cost $2000. As an electronic tech for the last 40 years I decided to attempt repair of control box. Took box apart and found 4 electrolitic caps on control bd. One 10 mfd 63 volt cap tested at 4 mfd. replaced bad cap, Unit now lights up first try, every try, now works fine. cost for new cap 20 cents.

Madison
November 30th, 2011, 06:29 PM
I have a Lennox Pulse that is 22 years old and I woke up to a 50 degree house.

Not knowing much about furnaces, I called a service tech and when he came it ran fine. So he pressure tested my heat exchanger, replaced the inlet diaphram, and everything was ok. He suggested if I keep having problems that I might need to replace the brain box at a $300 price tag just for the part. I decided to let it be and see how long it would run.

It failed 2 days later.

I didn't feel like making a $300 investment just for the part without trouble shooting and making sure thats what it was. So I decided I would do some basic trouble shooting. I had flame and heat in the furnace but it would turn off before the blower started.

I cleaned the spark plug (ignitor) and the flame sensor (both 22 years old, original, and made in the USA). Tried to operate and it failed but the furnace was hot so I know I had flame.

I turned the fan switch located my programmable thermostat from auto to the ON position then made the thermostat call for heat.

The furnace started and stayed on until the house got up to temperature and the thermostat no longer called for heat.

The Fan Limit switch is bad. The temperature inside the furnace reached the 200 degree limit because the switch wasn't turning on the blower fan to keep it in the safe operation range. When the Fan Limit switch hits the max it shuts down the burner to keep it from overheating and damaging the heat exchanger or worse, burning the house down.

It is possible that if you have problems with the flame not staying on that it may NOT be the flame sensor.

I ended up saving a bundle and learning a ton by doing a few basic trouble shooting steps. The fan limit switch is available online for about $115.

Teknician
March 3rd, 2013, 10:47 AM
Lennox gas furnace model #G12q3e-110-6 at my 88 year old mothers house probably 25 years old, with penn pilot ignition, would light for a second or two, then go out, repeatedly trying to light up. After several tries main burner would light and heat house. replaced heat sensor, no difference. Replacement ignition control box no longer manufactured. Updated control box out of stock for at least 3 weeks, cost $500 plus installation. Only option new furnace, cost $2000. As an electronic tech for the last 40 years I decided to attempt repair of control box. Took box apart and found 4 electrolitic caps on control bd. One 10 mfd 63 volt cap tested at 4 mfd. replaced bad cap, Unit now lights up first try, every try, now works fine. cost for new cap 20 cents.

I went ahead and pulled my board because my symptoms were, pilot relay (PV) seems to be coming on but the main burner (MV) wasn't. Also being an electronics and PC tek, I decided since I hadn't had any heat for 2 days and no call back from our local HVAC guy, I would pull the board and take a look.
Labeled everything, board came out nicely and pulled each capacitor at each relay feed 1 at a time. First cap read 10uf and resoldered it back in. 2nd cap read around 4uf so I replaced it.
Put everything back together and I'll be damned, IT'S WORKING LIKE NEW NOW!!!!!
Thank you Chuck, for the heads up!!
By the way, there is a new unit up on ebay in case anyone is interested...
Link removed