# Lennox Elite Furnace Issues (Slow blinking green light)



## Gideonr9 (Mar 28, 2018)

Hello everyone, 

Thank you in advance for reading this post. So I've got an issue with my upstairs furnace. The furnace is a Lennox Elite (M/N G51MP-36B-045-07). This thing has been giving me problems all winter. The error code is a single green slow flashing light. The sticker says "(A) Pressure switch open or, (B) Blocked inlet/exhaust vent or, Condensate line blocked or (D) Pressure switch closed prior to activation of combustion air inducer." 

I'll explain what I've tried, what I think started this issue, and then what I've been doing every time I need to fix it:

When this first happened I tried many different things.. I first had an HVAC technician look at it and he told me that my ecobee thermostat was causing the problem. I uninstalled the ecobee, returned it, bought a newer generation ecobee and installed it. Needless to say, it did not fix the issue. So I lost confidence in calling a technician and decided to do it myself. 

So I then tried to see if the pressure switches were bad, they aren't (I also just recently tried installing the pressure switch from the identical furnace downstairs that works, it made no difference). I made sure that the pressure switches were wired correctly, they seem to be. I tried draining the water from the little black box that is on the side of the furnace. I pulled off all the tubing that connected and made sure there were no blockages (water did drain out of some that were attached to the drainage system). I tried using a leaf blower to blow up the exhaust vent, I even vacuumed it just in case something was stuck there. I looked at and inspected the main control board for any signs of shortages or defects, I didn't see any.

Through the midst of all this troubleshooting I began to realize that occasionally the furnace would start up, the igniter would heat up, and just after the gas was ignited it would shut off as if there was an issue with the flame sensor. So I took out the flame sensor, cleaned it (it was really clean, to begin with) and put it back. (I never saw the other error code that signals this issue).

All of this troubleshooting happened over the course of 5-10 attempts to fix this stubborn furnace. Each time I just played around with it doing the things I described above and eventually it would continue working again. I really have no idea why it keeps doing this. 

Just recently I think I've found a pattern to "reset" it. It seems like after every time we get a lot of rain or moisture that it will stop working. Then I noticed that in order to fix it all I need to do is put my hand between the combustion air inducer motor and the exhaust vent/hosing/pipe.

I really am just fed up with this now and am debating just replacing the entire thing. I just want to be sure I've done everything to try and fix it first before I spend a few thousand replacing it. Especially if it's an easy fix that I seem to have overlooked. 

Can anyone give me more advice as to what they think might be the issue here? Thanks.


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## user_12345a (Nov 23, 2014)

Did you re-prime the trap after cleaning it?

Need a tech with a manometer to see what's going on.

could be so many things - i weak draft inducer could do it. A bad heat exchanger with popped crimp rings or really large cracks could do it.

could be something minor as well.

a bad t-stat can't cause this, neither will a dirty flame sensor.


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## Gideonr9 (Mar 28, 2018)

I primed the trap and it didn’t fix the issue. Although the same time I was up there doing that I turned it on again by doing the same thing I mentioned before and I can hear something rattling around inside of the furnace as if there were some sort of debris in there. 

Is it work pulling apart to see what it is? Or is that normal?


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## roughneck (Nov 28, 2014)

No, it’s not normal. 
What’s the draft reading when the inducers running, compared to the pressure switch rating?


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## Gideonr9 (Mar 28, 2018)

roughneck said:


> No, it’s not normal.
> What’s the draft reading when the inducers running, compared to the pressure switch rating?


I'm not sure. I do not have those fancy tools. But I installed a different pressure switch with a lower pressure rating and it didn't have any affect.


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## roughneck (Nov 28, 2014)

Why did you do that? You cannot just put in whatever rating you want. 
You could have possibly made your furnace unsafe. Put the original one back in unless the manufacturer says the lower rating is acceptable. 
You should be able to buy a micron gauge for a few hundred dollars.


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## ms52 (Dec 22, 2018)

I know this is an old thread but I'm new on the site, thought I would post my solution that fixed my Elite furnace. Tried all of the things you tried plus changing pressure switch, piping, and circuit board with no help. Then had a Lennox tech check it out and changed out the deteriorated 2nd stage box collector and solved the problem. Just a suggestion.


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