# Lennox 80MGF 3/4-100A - Working Intermittently



## Marty S. (Oct 31, 2009)

Flame sensor and burner cleaning should take care of the problem. Sounds like a weak flame signal fault then a watch guard( lock out) fault.


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## GStu (Nov 20, 2010)

*WatchGuard*

Hi Marty, 

Thx for the reply. Do all these units have WatchGuard? Are there other things to clean/check white I am it? Isn't there some sort of hose that get's clogged too or is that in the A/C part only?


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## yuri (Nov 29, 2008)

Post the complete model # and serial # so we know what series of unit you have. We are Lennox techs and the problems vary from one series to another.


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## GStu (Nov 20, 2010)

yuri said:


> Post the complete model # and serial # so we know what series of unit you have. We are Lennox techs and the problems vary from one series to another.


80MGF 3/4 - 100A 9
S/N 6398C 06703

Currently the furnace is turning on but not heating to desire temp. It's stopping about 1 degree shy but the whole thing shuts off at once instead of the flame then the blower. I would think the blower would continue for a little a bit while the heat exchanger cools right?


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## yuri (Nov 29, 2008)

Does it have the error code legend on the inner door or inside the outer door and tell us what codes it was giving you. Yours is a series 9
http://tech.lennoxintl.com/PDFs/9801b.pdf


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## GStu (Nov 20, 2010)

yuri said:


> Does it have the error code legend on the inner door or inside the outer door and tell us what codes it was giving you.


Error codes on inside wall. (pls see pics below).

Earlier today when it was doing the clicking the #2 LED was flashing and the #1 LED was off. It hasn't done the clcking thing again. Now it just works for a while but turns off after maybe 20 minutes. When restarting it now the #2 LED is solid and the #1 LED blinks but then both return to normal simultaneous blinking. I can post videos and/or pics.


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## yuri (Nov 29, 2008)

I can`t read upside down or those pics clearly.:no:

You need to look at those leds with a magnifying glass and make sure led 1 is itself as that board is upside down due the counterflow model. Should have the words above the leds.


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## GStu (Nov 20, 2010)

*oops*










I am identifying the LED's correctly. They are upside down but I checked at the board to make sure #1 was really #1, etc.


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## yuri (Nov 29, 2008)

Try clean the flame sensor. Attached to the white wire and by the left burners. Remove the burners so you can get at it. Blow them out with compressed air and clean the face of the one in front of the sensor with emory paper. Check where the tube from the pressure switch attaches to the ventor fan for blockage of the nipple it attaches to. Clear the port with a drill bit or paper clip, bit is better.


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## GStu (Nov 20, 2010)

I just cleaned flame sensor and all burners. Now it is doing what it was doing when it first started having problems. That is the thermo calls for heat, the inducer turns on and then the gas valve clicks on and off a bunch of times until everything stops and then this cycle repeats. Eventually it will get to where it keeps cycling but the valve clicks just once and then everything just shuts off. 

Below are 2 videos of exactly what's going on (just click on each pic). first video is the valve clicking cycle and 2nd video is when it doesn't click and everything ust shuts down and recycles. If you turn your sound up you should be able to hear the clicking. My wife doesn't manage to get the thermo turned up until about 35 seconds in.


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## yuri (Nov 29, 2008)

Now that is REALLY strange. I have worked on hundreds of them and not seen that problem. Could be 3 issues and I suspect the third one:
1) jumper R to W with wires removed on circuit board (after labeling them)Your thermostat may be faulty/cutting in and out quickly. If it runs well then the tstat is the problem.
2) shorting/faulty gas valve shorting to ground
3) faulty circuit board. Usually those last 10 yrs and yours is older. May have a bad gas valve relay. A difficult and VERY tricky board to change as you get a retrofit kit and only experienced Lennox techs can figure it out. It does several series and different models of units and if you miswire it or don't completely understand it major problems can occur. I would recommend you call an experienced Lennox tech.

NOT my favorite Lennox unit (will need a ventor fan soon etc) and I am quite pro Lennox. If you plan to stay in the house I would recommend a 95% efficient G61V (mutilposition) which I hear qualifies for rebates from your gov't but end soon. VERY good and reliable furnace the G61V, Marty has one I believe.


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## GStu (Nov 20, 2010)

yuri said:


> Now that is REALLY strange. I have worked on hundreds of them and not seen that problem. Could be 3 issues and I suspect the third one:
> 1) jumper R to W with wires removed on circuit board (after labeling them)Your thermostat may be faulty/cutting in and out quickly. If it runs well then the tstat is the problem.
> 2) shorting/faulty gas valve shorting to ground
> ...


So remove R and W and connect a wire between the two terminals? Do I turn power off first and then power on?

Is there any way to test for the valve short or bad valve relay? We are in no position to replace this entire unit just yet.


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## yuri (Nov 29, 2008)

Remove all the tstat wires and label them after shutting off power and then do the jumper job. Turn power on. The rest of the procedures I would have to do myself or a skilled tech with proper meters etc to test the valve etc. Depending where you live the fuel savings of the higher eff unit will pay it off as well as adding the $$ you don't have to spend repairing it will buy you quite a bit of furnace. Finance it thru the local utility co if they do that in your area.


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## BlaineM (Nov 19, 2019)

Hi nimunik, did you ever get this solved. This is likely a long shot as this is 10 years later, but if you are still active on this forum please let me know how you solved this as my furnace seems to be doing the exact same thing.


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## user_12345a (Nov 23, 2014)

why don't you start a new thread?


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## HVACJACK (Dec 31, 2020)

BlaineM said:


> Hi nimunik, did you ever get this solved. This is likely a long shot as this is 10 years later, but if you are still active on this forum please let me know how you solved this as my furnace seems to be doing the exact same thing.


Hi there did you ever get that fixed? Or did you start a thread somewhere? I had a no heat call today of this Lennox unit and it’s running intermittently when I first showed up the flame was lighting running for 10 seconds then going out inducer kept running and the same with the blower so I replaced the flame rod and igniter since that is the only kit they sell but now the unit runs for 4-7 minutes and is doing the same thing. I spent all day scratching my head testing stuff and still don’t have it running, can’t get through to Lennox tech support


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## roughneck (Nov 28, 2014)

HVACJACK said:


> Hi there did you ever get that fixed? Or did you start a thread somewhere? I had a no heat call today of this Lennox unit and it’s running intermittently when I first showed up the flame was lighting running for 10 seconds then going out inducer kept running and the same with the blower so I replaced the flame rod and igniter since that is the only kit they sell but now the unit runs for 4-7 minutes and is doing the same thing. I spent all day scratching my head testing stuff and still don’t have it running, can’t get through to Lennox tech support


What is the flame signal reading from your meter?


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