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I recently replaced my furnace control board. The furnace is

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a Lennox G40-OUH and the...
I recently replaced my furnace control board. The furnace is a Lennox G40-OUH and the control board was a White Rodgers 50-A65-121-07. The only replacement was a White Rodgers 50A65-843 Universal Board that replaces all old 50A65 ignition control boards. The replacement went fairly well except that now the furnace is not communicating with the thermostat. There are no trouble flashing indicators on the furnace and it works great when power is on but no temperature control to shut it down. Please help !
Thanks much!
Submitted: 6 months ago.Category: HVAC
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Answered in 1 minute by:
5/4/2017
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago
BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22,766
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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Hello,

Can you tell me what wires you have on what terminal (on the board) where the thermostat wires attach?

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Customer reply replied 6 months ago
Sure, I could just send you a picture if that would be easier?
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

thats fine...if you have ac also I will need to see those 2 wires as well at circuit board

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HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

you can attach any photos in this chat

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HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

thank you.

so when power is on...what runs?

blower

furnace/heat

ac?

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Customer reply replied 6 months ago
the blower and furnace/heat
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

if you remove the white wire from W...what does the unit do then when power on?

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Customer reply replied 6 months ago
I have not tried that
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

can you try now and let me know?

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Customer reply replied 6 months ago
ok give me a couple minutes
Customer reply replied 6 months ago
Looks like nothing, it did not come on.
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

thank you...sounds like thermostat shorted when board went, hook back up...then remove w at thermostat...if it does not run then you have a bad thermostat

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Customer reply replied 6 months ago
ok, would it have shorted another thermostat because I went and bought a new thermostat thinking that exact thing and it didnt work either.
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

I would know more once w is unhooked...

The thermostat connects R to W for heat...so somewhere there is a short..or bad stat...removing w from thermostat will tell us which

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Customer reply replied 6 months ago
ok will do
Customer reply replied 6 months ago
do i keep w disconnected at the board?
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

reconnect at board....then disconnect at thermostat

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Customer reply replied 6 months ago
ok thanks
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

any luck?

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Customer reply replied 6 months ago
sorry I can't find the new thermostat anywhere, I have the old one but I don't know if it is fried it was the one that we were using when I replaced the control board. Don't know if it matters but I have been using the heat for several days without a thermostat hooked up by just flipping the power switch to the furnace, is that a no no?
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

all I need is to have the white wire at thermostat not hooked to anything...or just remove the thermostat and tell me if the furnace still runs

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Customer reply replied 6 months ago
it does not, I had the wires attached to the thermostat plate without the thermostat on and it would turn on when I gave it power. now i just removed the white wire from the plate at the thermostat and it does not turn on.
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

then the white wire is clear and is only sending 24v to w on board due to a short at, in thermostat or wired wrong terminal.

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Customer reply replied 6 months ago
by terminal do you mean at the board? can you tell by the pictures if it is wired correctly, it is in the exact same position and color as the last one. and what do I do next?
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 6 months ago

your wiring at the board is fine.

the issue is there was/is a short at thermostat....or the thermostat is set for heat..

removed w wire and furnace stopped....this means somewhere at the thermostat the red wire is touching to where it energizes the white wire....after seeing the yellow wire and green wires at the circuit board it appears they got hot during a short circuit at sometime.

Short story = the reason the furnace keeps running is not because of the circuit board.....it is at the thermostat, thermostat wired wrong, thermostat wire pinched behind wall plate etc.....the cause is at the thermostat area

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Customer reply replied 6 months ago
ok because i hooked up the new thermostat if there is a short in the wiring could it have fried the new one as well?
or should it be ok? Also do I just go by the standard color code with the four wires as in the instructions of the thermostat?
Customer reply replied 6 months ago
are you still there?
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