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Old 11-11-2013, 09:14 PM   #1
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lennox 80 mfg house fan stays on


I am having problems with my Lennox 80MGF furnace.
The house blower runs and never shuts off.
The fan switch at the thermostat is set to "auto."
Diagnostic LED1 is flashing and LED2 is off.
This implies that the flame detector is sensing flame
while the gas valve is de-energized.

I've cleaned the flame detector.

When the house thermostat calls for heat the combustion blower
starts. Both LEDs flash. Then the gas lights and stays burning
until the thermostat is no loner calling for heat.
Then LED1 returns to flashing and LED2 is off and the house
blower continues to run.

I did not log what the house blower did during the start of the
heat cycle.

Here are some part numbers found on the controller board.
ECG2, 800A
HB00907DA V1F (on chip I think)
Cat# 73K8001
9713.

Thanks for any help.
Laplace888

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Last edited by laplace888; 11-11-2013 at 09:16 PM.
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Old 11-11-2013, 10:04 PM   #2
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lennox 80 mfg house fan stays on


remove the flame sensor wire at the circuit board if you can and turn on the power and see if the code goes away. if it does then the wire is shorting to ground. if it is in a multi wire harness then you cannot. if the sensor wire is frayed or shorted to metal then it will give that code and needs replacing. make sure the porcelain on the flame sensor is not cracked or black and if so it needs replacing. try cozyparts.com for Lennox parts

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Last edited by yuri; 11-11-2013 at 10:06 PM.
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Old 11-13-2013, 09:10 PM   #3
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lennox 80 mfg house fan stays on


yuri,
Thanks for your reply.
It would be difficult to remove the sensor wire from the board because it is in the multi connector plug.
I did measure the sensor resistance to ground (with the wire disconnected).
It was infinity.
If I apply power with the thermostat calling for heat the furnace will sometimes start. (The house fan starts immediately when power is applied)
The combustion blower starts and then the gas lights.

When I disconnect the flame sensor the furnace continues to work and both LEDs continue to flash.

When the thermostat no longer calls for heat the flame shuts off, LED1 flashes, LED2 is off and the house blower continues to run.


Thanks for your help
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Old 11-13-2013, 10:20 PM   #4
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lennox 80 mfg house fan stays on


Quote:
Originally Posted by laplace888 View Post
yuri,
Thanks for your reply.
It would be difficult to remove the sensor wire from the board because it is in the multi connector plug.
I did measure the sensor resistance to ground (with the wire disconnected).
It was infinity.
If I apply power with the thermostat calling for heat the furnace will sometimes start. (The house fan starts immediately when power is applied)
The combustion blower starts and then the gas lights.

When I disconnect the flame sensor the furnace continues to work and both LEDs continue to flash.

When the thermostat no longer calls for heat the flame shuts off, LED1 flashes, LED2 is off and the house blower continues to run.


Thanks for your help

This is INCREDIBLY dangerous and you could have a explosion and die. If the flame drops out and there is no working sensor then what shuts the gas off?? Sounds to me like the wire is shorted to ground and where it travels from the fan area up to the burners it probably is pinched and shorted there. DO NOT use the furnace until it is fixed. Stop horsing around with it as you are fooling the board with your startup sequence and that is very dangerous. Call a Lennox tech because if the circuit board is bad or now damaged the replacement may be a very complicated retrofit board/kit which the average tech will have a hard time with and is NOT DIY friendly. depends on the specific model and series and there are many of them in that model.
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Last edited by yuri; 11-13-2013 at 10:57 PM.
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Old 11-13-2013, 11:05 PM   #5
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lennox 80 mfg house fan stays on


Are you sure it is the house fan that immediately starts or is it the inducer (a relatively small fan in the same compartment as the burners). If the board thought the flame was still present it would give same symptoms.
By the furnace "still operates" with flame sensor disconnected do you mean there is still flame or do you mean things still move and it try's to light?
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Old 11-14-2013, 09:01 AM   #6
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lennox 80 mfg house fan stays on


Yuri,
Thanks again.

I agree, it is a very dangerous situation and the Lennox tech should be here in a few hours.

It appears to me that the flame detector has failed to a false flame detection mode. I don't know if the fault is latch in or would go away immediately when the fault condition was cleared. (no reset required)

I'm sure that is the house fan and not the combustion fan.


I'll let you know what the tech finds.
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Old 11-14-2013, 09:24 AM   #7
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lennox 80 mfg house fan stays on


Glad to hear it will be safe. The board could be failing and it is rare the wire shorts but anything is possible. I have replaced lots of those boards.
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Old 11-14-2013, 09:26 PM   #8
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lennox 80 mfg house fan stays on


Thanks all, esp Yuri!
The tech came. He did the necessary checks. Seemed to know what he was doing. I especially appreciated that he DID NOT take my word for anything.

He got a new board. Expensive!

It was pretty much an easy change out. All terminals seemed located at the same spot.

The furnace is working now.

Thanks,
Laplace888
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Old 11-15-2013, 09:40 AM   #9
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lennox 80 mfg house fan stays on


I don't take the customers word either, and have developed the skills of Sherlock Holmes to sniff out the truth. Half of what they say is true but at least that is useful.

the retrofit kit for the earlier models is a huge pain in the butt, full of extra wiring harnesses and wires that need chopping and reworking etc.

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Last edited by yuri; 11-15-2013 at 09:42 AM.
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