Service Forum for Lennox HVAC Owners and Servicers.
Enter Our Parts Store | Forum Home | Profile | Register | Active Topics | Members | Search | FAQ
Username:
Password:
Save Password
Forgot your Password?

 All Forums
 Repairing and Servicing Lennox Products
 Repairing / Servicing Lennox Gas Furnaces
 G43UF not working
 New Topic  Reply to Topic
 Printer Friendly
Next Page
Author Previous Topic Topic Next Topic
Page: of 2

CBASS

7 Posts

Posted - 12/18/2009 :  08:27:22 AM  Show Profile  Reply with Quote
I own a G43UF series furnace and found yesterday when I got home it wasn't working. Red and Green LEDs showed Alternating slow flash. Turned everything off and then back on. Furnace fan came on, and igniter lit the gas, but gas would turn off after 2 seconds. This cycle went on for 30 mins before the fan turned off and the LEDs went alternating again. Diagnostic codes say it went to Watchguard, burners failed to ignite or lost flame 5 times during a single heating demand. I cleaned the flame sensor, but still no luck. Anyone have any suggestions or know wht it could be?

Cozy.Support

5761 Posts

Posted - 12/18/2009 :  09:26:06 AM  Show Profile  Reply with Quote
Most likely cause is low flame current (but not the only possibility). A good place to start would be to clean or replace the flame sensor. Here is a link to the service manual for this model:
http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G43UF_09-01-2007.pdf



CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx
Go to Top of Page

CBASS

7 Posts

Posted - 12/18/2009 :  09:53:29 AM  Show Profile  Reply with Quote
Cool, thanks. I should have put the ohm meter to the flame sensor to check it last night when I cleaned it, but I will when I get home from work. Going to stop by the local Lennox dealer and pick up a new flame sensor anyway.
How would I check for low flame current?

Edited by - CBASS on 12/18/2009 09:54:50 AM
Go to Top of Page

Cozy.Support

5761 Posts

Posted - 12/18/2009 :  10:04:34 AM  Show Profile  Reply with Quote
If you have cleaned the flame sensor, this may not be your issue. Like was posted, there is other things that can cause the same code.

The average VOM cannot check flame current because many of them do not have the ability to measure a dc current this small. The flame current must be measured in DC micro amps by placing the VOM leads in series with the flame sensor.

Did the burners ever ignite, even for a few seconds?



CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx
Go to Top of Page

CBASS

7 Posts

Posted - 12/18/2009 :  10:35:10 AM  Show Profile  Reply with Quote
Yes, burners would always ignite, but only stay on for 2s and then turn off. This cycle would repeat for about 30mins and then the combustion air inducer would turn off.
Go to Top of Page

slctech

2085 Posts

Posted - 12/18/2009 :  8:44:10 PM  Show Profile  Reply with Quote
I would be suspicious of the pressure switch dropping out. Could be border-line pressure closing it upon initial call for heat but once a heat load is introduced it drops out, check for continuity through the pressure switch and see if is dropping out once the burners ignite. If it is not the pressure switch and it is holding steady continuity, then need to read the micro-amp readings produced by the flame sensor should be around 2 micro-amps with a bare minimum of about .5 micro-amps. If the flame sensor is putting out a good signal and the pressure switch is holding, then check voltage to gas valve, should have a constant 24v across valve terminals, if voltage is dropping out then I would be suspect of the board, if the voltage stays steady and burners are dropping out then I would be suspect of a faulty valve....welcome to the world of being a service technician.
Here is the service manual for your unit: http://tech.lennoxintl.com/PDFs/0416h.pdf

As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!

Edited by - slctech on 12/18/2009 8:51:56 PM
Go to Top of Page

CBASS

7 Posts

Posted - 12/18/2009 :  10:52:52 PM  Show Profile  Reply with Quote
Measured voltage to the valve, steady 25V when burners on. Continuity on the pressure switch was good until the valve turned the gas off and continuity went off zero, but then went back to zero. Flame sensor was at zero until burners came on. Got to .2 micro-amps before burners shut off. This is the 2nd flame sensor. I'm stumped, checked for obstructions on the intake pipe and it was clean.
Go to Top of Page

Cozy.Support

5761 Posts

Posted - 12/18/2009 :  11:30:59 PM  Show Profile  Reply with Quote
Pressure switches that are getting a marginal pressure will generally open soon after the burner ignites. The reason is because once the air starts to heat the density of the air inside of the induced draft blower drops and the pressure going to the switch will also drop. As soon as the burner kicks off, the air passing through the induced daft blower will begin to cool, getting denser, and then the switch will close again.



CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.

Once you know what part you need, you can contact the Cozy parts department by using the link below
http://www.cozyparts.com/Lennox-Parts/locator_form.aspx
Go to Top of Page

slctech

2085 Posts

Posted - 12/18/2009 :  11:58:47 PM  Show Profile  Reply with Quote
If your actually reading your flame sensor properly, .2 micro-amps is your issue. .18 is the published drop out voltage for the board. Need to look at the flame sensor and see if it positioned properly and check all your ground connections especially between the burners and the chasis/metal body of furnace. Pull and clean the burners with a wire brush especially the burner corresponding with the flame sensor. What ever you can do to get the flame sensor reading at least above 1 micro-amp.
I have been doing service for almost twenty years and in those twenty years I have only replaced about ten flame sensors, of those eighty percent were due to poorly adjusted over-firing propane furnaces, the other twenty were due to broken/cracked ceramic bases...what I am saying is that a flame sensor is a die-hard component and there is usually always something causing poor rectification other than the flame sensor itself if it is clean.

As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!
Go to Top of Page

CBASS

7 Posts

Posted - 12/19/2009 :  07:30:56 AM  Show Profile  Reply with Quote
I will double check the flame sensor. It is located properly, but my question is, if the gas is only on for 2-3 seconds, can the flame sensor reach proper amp reading?
Thanks for all your help.
Go to Top of Page

slctech

2085 Posts

Posted - 12/19/2009 :  11:08:50 AM  Show Profile  Reply with Quote
Yes, 2 or 3 seconds is the usual alotted time for the board to sense the flames. Flame sense requires a solid ground to complete the rectification process. Do research on-line on flame rectification to get a full understanding of this process. It is not the theory of warming up of the flame sensor that increases the electricity, the signal is already being produced, board is just waiting to receive it back.
Here is a good place to start: http://hvacwebtech.com/Flame%20Rectification.htm


As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!
Go to Top of Page

CBASS

7 Posts

Posted - 12/19/2009 :  4:39:24 PM  Show Profile  Reply with Quote
Flame sensor does measure 1.6 micro amps but still have the same issue.
Go to Top of Page

slctech

2085 Posts

Posted - 12/19/2009 :  5:11:40 PM  Show Profile  Reply with Quote
Now that you have a solid signal from the flame sensor, now you need to check and triple check your pressure switch and gas valve and other limits before even thinking about condemning the board. And check you have a good solid ground from the burners back to the board.


As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!

Edited by - slctech on 12/19/2009 5:13:26 PM
Go to Top of Page

slctech

2085 Posts

Posted - 12/19/2009 :  5:31:29 PM  Show Profile  Reply with Quote
Re-reading your previous post about your voltage readings at the gas valve. You stated that you lost continuity as soon as your gas valve lost voltage. As long as the inducer motor is operating then you should have continuity across the pressure switch whether or not you have power to the gas valve. That statement of yours leads me to believe that you have either a pressure switch issue, clogged pressure switch tubing, clogged drain tubing or assembly, clogged or restricted ports which the tubing attaches to.
Definitely not the gas valve if your voltage is dropping out to it, to soon for other limits to be an issue, must be a pressure switch issue or a ground issue...verify that both are DEFINITELY not the issue, then it would be a board problem. If and when the unit locks-out after numerous attempts what are the diagnostic lights on the board saying?

Your pressure switch is reading differential pressure between the combustion air of the burner box coming in and the venting system, and restrictions in any side can cause issues. Inclinometer or a dual port manometer is required to 100% verify if there is a pressure switch or internal venting/combustion issue.

As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!

Edited by - slctech on 12/19/2009 5:39:25 PM
Go to Top of Page

CBASS

7 Posts

Posted - 12/20/2009 :  09:56:18 AM  Show Profile  Reply with Quote
Clean the lines for the drains. One had some deposits built up in it. After that furnce works great. Thank you all for your help.
Go to Top of Page

slctech

2085 Posts

Posted - 12/20/2009 :  09:59:43 AM  Show Profile  Reply with Quote
Great Job!

As a Lennox Technician I feel basic repairs and maintenance can be performed by the homeowner. But I highly suggest that periodic maintenance should be made by a certified licensed technician. You should contact your local Lennox technician for any repairs or issues that are outside your comfort zone. Property damage, personal injury, and death can occur from improperly repaired or maintained systems. Replacing a part doesn't always resolve the issue that caused the part to fail!
Go to Top of Page
Page: of 2 Previous Topic Topic Next Topic  
Next Page
 New Topic  Reply to Topic
 Printer Friendly
Jump To:
Service Forum for Lennox HVAC Owners and Servicers. © 2002-2013 CozyParts.com, Inc Go To Top Of Page
This page was generated in 0.11 seconds. Snitz Forums 2000