Lennox Gas Furnace locks out...stuck on purge

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Old 02-03-09, 11:36 PM
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Lennox Gas Furnace locks out...stuck on purge

2 1/2 yr old Lennox Gas Furnace gets signal to heat and the purge starts and goes thru total heat cycle and shutdown, except that the ending purge fan (combustion air inducer) just stays on. The blower is for sure off, along with the gas. The green light is blinking once a second and red light is off. I have water in the floor catch pan. The tech came and re-glued pvc to stop small leak, which I did not think it was leaking from. Anyway, now I have the system working with the clean-out access cover of the condensate trap off so it can drain and so far it is working without problems. You would think that it would drain down the sump, but pulling the cap off and looking in shows that it is faulting without even reaching the pipe to drain the water down the sump. It is producing 4-6oz of water a cycle. I have had two techs come in and all I got was $150 lighter. Thanks for any help.
 
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Old 02-04-09, 01:06 AM
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You seem to be describing two different problems, but discussing them together suggests you think there is a connection between the two. Is that the case?

An inducer motor that continues to run implies a bad circuit board that is failing to shut it off.
 
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Old 02-04-09, 11:57 AM
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Possibly it is two problems, I really don't know. As of this am, my little test has failed. The inducer motor kept running after the heat cycle. Again, DS2 Green light was slow blinking with DS1 Red off. Strangely, ALOT of water drained out after I recycled the system (pulled off lower cover then replaced it). It also sounded like the water was bubbling, (before i pulled the cover off and while the inducer was still running), as if trying to go against the flow of air. The fault code is read as being one of three or four possibilities. If it is the mother board, you would think that the father board would get her s**t squared away
 
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Old 02-04-09, 02:50 PM
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Please post the model number of the furnace from the rating plate usually found in the burner compartment.

Also post just what the diagnostic code lists as possible problems.

At least with some Lennox furnaces that code would indicate either a pressure switch opening or a blocked drain, which you apparently have.
 
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Old 02-04-09, 05:12 PM
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Make sure the furnace really did reach the end of the heat cycle. It may not have.

If a pressure switch opens, the flame goes out, just like how it does at the end of the heat cycle. The thing is, at the end of the heat cycle, the inducer fan is supposed to quit also.

But you could be fooled into thinking the heat cycle ended, when in reality it may not have. And instead, the pressure switch opened, which will only usually kill the flame, but leave the inducer on, if there is stil a call for heat.

IF it really reached the end of the heating cycle, you could verify that by testing W terminal (on the 24 volt power strip (inside the furnace, that has other skinny low volt wires hooked to it, and usually has abbreviated letters, like R,W,Y,G,O,? to ground. If that circuit pathway (W terminal to ground) is dead, that would indeed indicate the call for heat ended - and then I'd have to agree that a relay is stuck, and that may be located on your control board. But you would have to make sure of that, also, before blindly buying a board.
 
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Old 02-05-09, 08:09 PM
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It is a Lennox G51MP Series Gas Furnace The fault code suggests 1ressure switch open 2:blocked inlet/exhaust vent 3:condensate line blocked or 4ressure switch closed prior to activation of combustion air inducer. Hope that helps.
 
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Old 02-05-09, 08:13 PM
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So looking at my last post it seems when you put a : and a p near each other it becomes an emoticon. How fun, sorry if it confuses.
 

Last edited by f1fan; 02-05-09 at 08:24 PM. Reason: wrong verb tense
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Old 02-05-09, 11:32 PM
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The manual for your furnace is here:

http://www.cozyparts.com/Lennox-Part...09-01-2007.pdf


On page 44 the diagnostic code you report is described. The Red LED light is #1 and the green LED light is #2

You report #1 off and #2 a slow blink.

On page 44, the furnace operates normally to the following point

"Has combustion air proving switch closed in 2.5 minutes"

Answer: no. That causes the diagnmostic code you report to be displayed


On page 13 this is described in a little more detail:

"Once the proving switch is determined to be open the combustion air inducer is energized.... If the prove switch is not proven withing 2 1/2 minutes the control goes into a 5 minute prove switch lockout period."



Anyway, it looks like that is where you are stuck: the pressure switch isn't closing because something is plugged up in the furnace, one way or another.
 
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Old 02-06-09, 03:49 PM
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Thanks for the info. After that 2.5 min period should the inducer turn off? Because it doesn't, it just keeps going until we manually reset the system by taking the front cover off. My worry is that we are going to fry something, like our entire house. Again, thanks for the help.
 
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Old 02-06-09, 04:12 PM
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I garee that there is a discrepancy between what the manual says should be happening and what your report, with the inducer motor continuing to run.

There could be explanations for that, such as you might have a different circuit board than the one that's being described. The one described is circuit board 69m15. What one do you have? If it's different it may have variations such as the one you describe.


But you aren't dependent on the diagnostic codes. You should double check the idiot lights by checking to see if the pressure switch is in fact closing when it should. If it is and the circuit board isn't responding to that fact, let me know.

If it is open, you still have to deal with that fact. You could also jumper out the pressure switch when it should close to confirm that the circuit board then continues to light the furnace properly.

There are several ways to verify what is happening.
 
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Old 11-22-11, 05:45 AM
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Red face Lennox G51PM

I had problems with my G51 going through 1 cycle of heat[reaching Thermostat setpoint] The Gas supply would shut off, the blower motor would shut off, but the Combustion motor would not shut off. It would get very hot. I took the front panel off, with the power safety shut off switch,which turned off power to Comb. Motor,and PC Board etc. I would let the comb. motor cool off, then hold in the safety shut off switch. The power board red/green lEDs would fast flash, and the gas igniter light up, the gas flow valve would open, supplying gas to the burners,along with the burner motor. After 45 sec or so,after the flame ignition, the Blower fan would come on. After this,I would release the Safety button[which shut off the power,gas etc.,,replace the panel, and the cycle would start up and continue until the Temp setpoint reached,then once again the same thing would occur. After going through this a few times, I thought I would move the position of the Shutoff safety switch outwards,just a fraction, to see if it was the cause. As I suspected it was! The furnace is now working fine. Didn't even have to replace this switch. Hope this helps someone. It was driving me crazy!!
 
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