Lennox furnace problem

[doctorair]

I'm on a Lennox G40UH-48B-090 furnace that is driving me crazy.
1st call- Flame would come on for 3 seconds then shut off. Cleaned flame sensor and unit started working great.
2nd call about a week later- Same problem. Went ahead and replaced sensor and cleaned the ground real good. Homeowner poured bleach down drain this last summer and spilled some inside of furnace. The ground was all rusted up. checked gas pressure (natural) and its good.
Unit started working great again.
3rd call another week later- same problem. I show up to a no flame error and blower is running all the time. I replaced the circuit board this time ( warranty).
Unit works fine. Cycled it and watched it for 30 minutes without a hiccup.
Today- They called back and said its doing it again. They are getting a no flame error. They said they unplug it and it will start running fine. Seems to go out in the middle of the night.
I'm stumped now. New flame sensor and circuit board. I know Lennox has had problems with the pressure switch hose but wouldn't that be a different error? All safeties seem to be ok. Why low or no flame signal?

[Cosmicmuffin]

Is this a closed combustion unit. I came behind someone that had installed a universal ignitor, Same problem.,called tech support! They said they had problems with flames "swirling" and not sensing., intermitant. I thought they were nuts.,but with no other alternatives. Changed out ignitor with new gasket,Problem solved. Check for gromets missing etc.
I'm on a Lennox G40UH-48B-090 furnace that is driving me crazy.
1st call- Flame would come on for 3 seconds then shut off. Cleaned flame sensor and unit started working great.
2nd call about a week later- Same problem. Went ahead and replaced sensor and cleaned the ground real good. Homeowner poured bleach down drain this last summer and spilled some inside of furnace. The ground was all rusted up. checked gas pressure (natural) and its good.
Unit started working great again.
3rd call another week later- same problem. I show up to a no flame error and blower is running all the time. I replaced the circuit board this time ( warranty).
Unit works fine. Cycled it and watched it for 30 minutes without a hiccup.
Today- They called back and said its doing it again. They are getting a no flame error. They said they unplug it and it will start running fine. Seems to go out in the middle of the night.
I'm stumped now. New flame sensor and circuit board. I know Lennox has had problems with the pressure switch hose but wouldn't that be a different error? All safeties seem to be ok. Why low or no flame signal?

[wraujr]

Had one where burner needed a good cleaning, especially the crossover portion as the flame wasn't crossing to the third burner fast enough. Every time I cleaned sensor it worked for a while, I guess because the sensor was temporarily more sensitive. After cleaning crossover, worked great for years with the same flame sensor....

[martyinlincoln]

Most Lennox furnace will give intermittent flame sense faults once you get below .7 microamps . What was your reading? Saw you cleaned and then replaced flame sensor and cleaned the grounds. How about the burner face?

[Makinhole]

clean the burner. I bet the problem will not come back. :cheers:

[JBM1000]

All of the previous suggestions plus one more, have very rarely had a "Bouncing-Betty" pressure switch just fast enough to not be recognized by control board as psi failure but dumps to flame failure or watch-guard status.

Plus believe it or not the HSI is actually a flame sensor back to the E-prom chip on control board and has to have a certain value to maintain ignition regardless of the flame sensor circuit. "This is direct from a Lennox Nerd Engineer". So its OEM every time now when I do a major brand with any struggling problems of age etc.

[talktowags]

:ditto: Had Bryant with same symptoms. HSI was glowing but not getting hot enough to ignite gas ( intermitten ):gah:

[iszell]

You say it seems to go out in the middle of the the night, I found at a house before that the main gas pressure regulator was going bad, and at night when none of the other appliances was operating in the house (hot water heater), the furnace would stop working (wouldn't ignite). So you might have a creeping pressure regulator. Ask the home owners do they have hot water in the mornings, if not then there you go. Also try adjusting the ignitor to align in front of the burner if you haven't already

[SuperTech2010]

has it been raining lately? check the pressure switch hoses...check the flue pipe...i had a bee's nest drive me nuts a week ago....if there is NO code then i feel like its a pressure switch cutting in and out....make sure it also not water in inducer, or in flue pipe.....

[SuperTech2010]

tell the homeowner to stop turning the power OFF so when you get there it will coded out...and i am bad for this also>>>> don't take the door off before looking to see what the code is..... i am sticking with a pressure switch, maybe even something making it stop.....there is a reason its shutting down

[Hemi348]

I've had a similar problem with a lennox 80% and diagnosed it to the pressure switch jumping back and forth so quick my meter had a hard time picking up the voltage. But it had the same symptom as yours.

[doctorair]

I went back to call and everything was working. Called tech support and they seem to think it could be a pressure switch bouncing back and forth. All very good comments and will definitely check them also. Thanks guys, I'll keep you updated

[suBcools]

I also vote pressure switch. I had an 80% lennox giving me a similar problem last winter. It was throwing low flame sense or no flame codes. I cleaned flame sensor, cleaned burners, cleaned grounds, and watched it run perfectly for about 45 minutes. I got a call back about 2 days later. It turned out to be a bad pressure switch. I used my aligator ends for my test leads, and clipped right to the pressure switch as it was running, and just sat there and watched my meter. After about 25 minutes, the flames dropped out for a second, then it immediately went to re-light. It happened so fast that my meter barely caught it. I was watching it sit on 0 volts, then for a split second, it started to register some voltage but never even put out a full reading. The numbers popped up on the meter for a sec, and went back to zero. I sat there another 20 minutes and watched it happen again. I replaced the pressure switch and never heard back from them again.

I think the reason it was giving no flame/ low flame sense code is because when the flames would drop out, the pressure switch would close back up so fast (nearly instantaneously opened and closed right back up) that the gas valve was trying to open again right after the flames dropped out, but it wouldn't light, thus causing the board to think no flame. Then it would immediately energize the HSI and retry ignition and light successfully. Surely after that happened 3 or 4 times in a period of time it would lock out.

[Bsmith816]

Ran into same problem with a g40 Lennox furnace a few weeks ago. Kept showing low flame signal. Need to remove manifold and take out orifices and clean them. Found a spider living in there and would sometimes block the orifice. Another tech was out there before me twice. After I cleaned and reassembled I haven't been back since.

[Bsmith816]

You should also have your magnahelic hooked up to your pressure switch to watch and see what the differential pressure is and see if it's fluctuating much. I bet you have a clogged orifice though. There is a bug in there giving you trouble

[BigBacardi]

LOL...my first post .....this a lennox diplomat?

appears to be going out on flame rod...3 seconds...

this is what is wrong...fer real....inducer wheel is dirty....

what happens is the air becomes more fluid when hot, and slippery.

i promise you....clean the wheel and problem solved. prove it to yourself by putting manometer on the pressure switch tubing and jump out switch...watch the pressure drop.

[HomesteadHVAC]

I had this problem on a Lennox last year. Frustrating because it can make us look bad when we say it is fixed, bill the owner, and get a call back.

I finally decided to call Lennox tech support and he advised me of a problem with the pressure switch opening for a split second. I doubted it at first but once I ran the furnace with my voltmeter connected, I saw it with my own eyes.

I recommend doing the same if you have the patience to wait for the fault to happen, or changing it out and seeing if the phone rings a year from now.

[BigBacardi]

And like I said.....the switch is opening cause the wheel is dirty....