Gas Furnace - Limit Switch?

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Old 05-12-20, 07:44 AM
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Gas Furnace - Limit Switch?

Hello,
I have a York FG8B07512MU11A gas furnace for my upstairs heat. There have been many nights where my family and I go to bed with the heat working, and we wake up in a cold house. When this happens, the control board has a red light that flashes 4 times, indicating a limit switch open. To get the system to turn back on, all I have to do is press on or move the control board (anywhere on the board). This causes a clicking noise (relay?) and the burners will then ignite. This happens at least every other night. Any ideas? Do I have to replace the entire control board? See video below:

https://www.youtube.com/watch?v=qsLX_XJVdc4

Thank you,
Craig
 
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Old 05-12-20, 08:11 AM
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Looks like you have a bad solder joint or a cracked copper track on the board.

So it all depends if you have a soldering iron/gun and some experience soldering electronic parts.
 
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Old 05-12-20, 09:28 AM
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Is there a way to confirm this? Do I take the board off an visually inspect for cracks?

Thank you,
 
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Old 05-12-20, 09:45 AM
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Yes shut all power off to the unit and remove the board.
Take note of where the wires go. Usually they can only go in one place re: different connectors but that is not always the case so take pictures and make notes/diagrams, mark wires etc.

Then look at the solder side of the board.
A magnifying glass and a strong light help here.
Bad solder joints are often a duller silver color and/or have pin holes.
Check for cracks in the copper trails while gently flexing the board.
While looking at the solder side gently wiggle various components to see if the component leg moves in the solder or the copper connect tab moves.

A very common place for bad connections is the solder connections at the relays.

If I cannot locate the bad joint I will re-melt/solder most connections on the board with my solder iron usually adding a little fresh solder where I can.
 
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Old 05-12-20, 10:38 PM
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Welcome to the forums.

Unfortunately it is not recommended to repair furnace control boards due to the possibility of disabling a protection circuit during the course of repair. Highly recommend replacing it.

Control Board S1-331-03010-000
Control Board S1-331-03010-000
 
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Old 05-14-20, 06:19 AM
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Thank you for the information. Replacing the control board seems straight forward. Are there any other checks that I should do before ordering one? Or, does the description of my problem and the way that fixes the problem confirm that something on the board is bad?
 
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Old 05-14-20, 09:32 PM
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The only other problem that is fairly common is that 9-12 pin connector gets corroded.
You could try unplugging it and plugging back in a few times to see if that changes anything.
 
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