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Lennox furnace problem

6383 Views 17 Replies 4 Participants Last post by  trandothuy
Hello, I am new to this forum. I found the forum while searching for my Lennox G61MPV problem. I don't want to start a new thread and I don't want to hijack OP but I have the same unit. If I need to start a new thread, let me know. My diag code is flashing 2+3 which is low pressure failed open. My unit sits horizontally.

Exhaust vent to top of roof. Intake pipe is open to attic space. There seem to be some debris in the exhaust pipe. I took a 15' flexible electrical fish rod and shove it down the exhaust flue pipe from the roof. I had a small container on the other end to catch the debris. I removed the many rubber tubes inside the unit and blow it out with compressed air and brown gunk comes flying out of tee pipe.

I have many questions and hope someone will educate me on this unit. I am not HVAC guy so any explanation will be greatly appreciated.

While searching on the internet, I see many folks with Lennox high efficiency unit show a diag code of 2+4 (High pressure switch). Mine show a 2+3 code (Low pressure switch).

Could some one educate me on what the low pressure switch regulates and what the high pressure switch regulates?

Second question, the black box trap, does it have a switch or float inside this black box?

Third question, does the black box need to have some water in it for the heat furnace to run?

Btw, AC run fine Or (atleast that what it appears to me).

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I was wondering where my post was. Thanks.

It will be cold tomorrow and I am still without heat. I am working on this real slow (have full time job). Because my exhaust pipe goes to the roof, it is at least 15' + and slope so I cannot really tell for sure if I clear all the debris. I want to toss something down the exhaust pipe to see if it fall all the way to the bottom but I don't want any more un-necessary object lodge inside the pipe if indeed there is a blockage.

The unit is only 6 years old. If I can figure a way to check the long flue pipe for blockage and can 100% sure there is no blockage and the thing still does not run I will have to call Lennox directly and asked for their recommendation on a technician.

The reason for getting a Lennox tech out is to also asked for his/her opinion on the way the exhaust is vented to see if it is correct and why the builder never place a cap on the end to prevent things from getting into the exhaust pipe. Also to asked if the drain (black box area) is properly installed and if the furnace is tilled properly.

The external pipe attached to black box is insulated so I cannot tell if there is a trap there. I don't want to cut the insulation at this point. It does get very cold in the attic during winter time down south here in Texas.
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I have checked the opening in the collector box and used a ear swap to poke it. I am not sure what is in the collector box so I used a q-tip to clean it.

I will check again. I noticed you say one at a time? Is there a reason for removing one tube at a time? When I cleaned mine I removed both red and black tubes. Did that cause any issue that I am not aware of?

I checked the port. I do see the + and - on the switch and collector box, they are connected correctly.

For the positive, a 2" nail goes all the way in. For the negative port, the 2" nail goes in 3/4 and hit something like metal. This is same for both low and high pressure ports.

Still does not run and flashes the same low pressure failed open code. Should the nail goes in the same length for + and - ?
No new update. I did the best I know how to see if the exhaust is completely blocked and it was not blocked (could be partially block but kinda difficult for me to confirm without cutting the pipe).

A manometer would be nice to check the pressure switch. I am tire and feel defeated at the moment. Will let you all know if I have any real update. Thank all for your valuable inputs.
Hi, I did open the clean-out port and blew out lots of fluid and gunk with compressed air so I know it is empty. I also read in the installation manual and they say to prime condensate trap and follow the 6 steps. The inducer will not run so I cannot prime it based on the instruction.

I have called Lennox and they gave me 2 company in my area that is Lennox certified so I have called one of the company and scheduled for them to come out to troubleshoot. I also checked the BBB and they are in good standing with 1 complains.

Thanks everyone for all your inputs and educating me on Lennox gas furnace. Hopefully I will have the furnace running again without too much damage to the wallet.
Update for anyone reading. My inducer draft motor is the culprit. This time the technician explain to me why the motor need replacement and why the code say low pressure failed open. But does anyone know what so special about this inducer motor and why the code say low pressure failed open? Hint: It has 4 wires coming out of it. Red, black, white, green.

I was told this is a very unusual thing for the motor to crap out given the age of this unit. A very costly thing to replace. But I googled and the motor cost around $400. It is amazing how much labor is on this motor. I have removed the motor many times to get to the hose to clean the hoses and it not that difficult to remove.
Yuri, the unit is 6 yrs old.

This inducer motor is a 2 stage motor. Hi and low. The low stage for the motor is not working. That is why I think I got the Low pressure switch reading failed open and the motor will not kick off when call for heat.

Who really know why the low stage failed but like you said could be all that debris making the motor work more than necessary.

Although not recommended, I can get the motor running on only 1 stage (high) and get some heat in the house for now.

This is an expansive item I have no fund for it right now. Just got out of the hospital for a procedure and paying 20% of that. Thank god for health insurance.
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