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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22508
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Lennox G27M5-120A-2 furnace. LED flashing codes

Resolved Question:

I have a Lennox G27M5-120A-2 furnace. LED flashing codes that indicated:
1. Secondary limit was open
2. Pressure switch was open

Additionally blower was not cycling on in normal fashion.

Troubleshooting revealed that the secondary limits were operating properly, the pressure switchs were operating properly. Problem was that a relay on the SureLight circuit board blew out and destroyed back of the board.

I replaced the surelite control board but LEDs still indicate that the pressure switch is open. Also the exhaust fan runs continuously when the furnace is powered on, holding the pressure switches closed continuously.

When the leds indicate the pressure switch is open I can clear this code by removing the hose (opening the pressure switches) then replacing the hose (closing the pressure switches). After this, the furnace works properly until next cycle.

I'm thinking it must be the other control board, but no visible indication of dama
Submitted: 7 years ago.
Category: HVAC
Expert:  M Oglesby replied 7 years ago.

It sounds like you have a bad pressure switch. I recommend you replace the pressure switch on your unit.


Please hit ACCEPT so I can be paid for my time. Thank you very much!

Customer: replied 7 years ago.

Tell me more as I'm not following your logic. Both pressure swithes look to me to be operating ok. What I mean by this is that the switches are both closed as result of the vacuum applied by exhaust and when I remove the hose temporarily when I get the error codes... allowing both switches to open temporarily and then close again... this action gets the unit running until the next cycle. I come to this conclusion by montioring continuity in the circuit during the cycle. So the pressure swithes hold themselves closed. Why do you think the switches are bad?


Also.. why is the exhaust fan running constantly? Since my manual intervention of removing the hose seems to fix the problem, I was guessing that the furnace is supposed to be doing this itself automatically by cycling the exhaust on and off.

Expert:  BillyHvac replied 7 years ago.


Hello, the issue is a stuck inducer relayon the inducer control board. This in turn is what is holding pressure switch making furnace code fault.



Customer: replied 7 years ago.
Thanks Billy. So I'm hoping this relay is on the OTHER board.. ie, not the SureLite control board that I just replaced yesterday. So what's the fix Billy?
Expert:  BillyHvac replied 7 years ago.


You are correct. The issue is now with the 2 stage control board.


It is not uncommon for a domino effect to take out the sister board like that.


For you r convenience I am including the service manual for you. Page 4 will show the 2 stage control board location.


Thank you for your patience.

BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22508
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
BillyHvac and 4 other HVAC Specialists are ready to help you
Customer: replied 7 years ago.

Thanks Billy. I also found the service manuals earlier in the week... very handy. I have just two more questions for you as long as I've got your attention:

1. Do you have a recommended source for parts.. one you like more than others?

2. As long as I'm ordering parts, any other preventive maintenance items you would suggest replacing as well?


Thanks again for your help.

Expert:  BillyHvac replied 7 years ago.


I like this outfit for parts -


As far as other preventive items, this unit really has no "routine" maintenance parts to change. I would change the board, clean blower compartment, remove vent pipe and shop vac unit out, and make sure of fresh filter.