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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8067
Experience:  Furnace, A/C and heat pump specialist.
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I have a Lennox G61MP furnace that stopped heating. I

Customer Question

I have a Lennox G61MP furnace that stopped heating. I believe it was due to a dirty filter. I replaced the filter. Not sure how to get it going again if the old filter caused the limit control to shut the unit off..?
JA: Thanks. Can you give me any more details about your issue?
Customer: That's it.
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Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

Did you get a limit error on board?

Customer: replied 1 year ago.
I am not yet sure. At work now and have not checked the board, and have no experience reading the error codes on the board.
Expert:  airheatman replied 1 year ago.

If furnace shut down because of dirty filter,you may have two limits on back of blower housing that trip and a main limit in furnace vestibule.Most of these are auto reset and will rest themselves once cooled down,Some have a small button in center that pushes in to reset.When you get home,Lets turn power off and back on to furnace .Then call for heat and see if furnace will light or what error code we get by light on board.

I will help you diagnose the code.Just let me know what lights do for example 3 and 1 indicates an open limit.

We can go from here.


We would may a meter to troubleshoot. You can get an inexpensive one at Walmart for about 15 bucks if you would like to troubleshoot and I would be happy to guide you.

Customer: replied 1 year ago.
Ok, thanks. Will follow those instructions and see what code I get and let you know. Thanks.
Expert:  airheatman replied 1 year ago.

Sound good, we will go from here

Customer: replied 1 year ago.
I checked the board and it is signaling a pulse - 1/4 second flash followed by 4 seconds of off time - which the manual says means "power on - Standby". Should it be working?
Customer: replied 1 year ago.
I pressed reset on the thermostat and tried to turn on the heat. It did not come on and now the board is signaling 2+4, which reads as "low pressure switch failed closed".
Expert:  airheatman replied 1 year ago.

On call for heat did the small inducer vent motor come on?

Customer: replied 1 year ago.
No, nothing came on when I called for heat. At least not when the fan is set for auto.
Expert:  airheatman replied 1 year ago.

Ok do you have a volt meter to see if the vent motor is getting power?That is the very first thing that has to happen so it will draw a vacuum on pressure switch.

Customer: replied 1 year ago.
I don't. Will get one tomorrow.
Expert:  airheatman replied 1 year ago.

Excellent. reset power to furnace and call for heat,Then see if we get 120v to this small vent motor.We will go from here.

Customer: replied 1 year ago.
It is working now. Thanks.
Expert:  airheatman replied 1 year ago.

If you have further questions, please let me know.

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