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Hello, tell me a lot about your skill sets with electrical things, so I can coach you most effectively please.
I am back, there is trouble on the internet today that is cause our JA server problems and I think more broadly as well, what I type is often erased before I can send it, so I will send short snippets, that should work
Send me a photo of the thermostat wire connections inside the furnace blower compartment... there should be small red, white, yellow, green, and blue wires, probably more, I can mark that up with some initial trouble shooting directions
Next I will probably need a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.
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I can mark it up with trouble shooting instructions.
We can go from there.
Thanks, ***** ***** in my 51 year career and thousands of repairs this is the first time I have seen 4 wires attached to most of the terminals... This could be something that will not be amenable to diagnosis on the web.
I suspect a bad printed circuit card however,
We can do a heat stress test on it
Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has an invisibly cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.
No change however does not prove the card is good however... but a change proves the card is definitely bad.
That is not how dual systems are traditionally wired at all, your furnace could be an exception, but it would be a first in 50 year experience. get me the wiring diagram from the inside of the blower service panel I will look it over. and do the heat stress test,
We can go from there
No error codes is why I suspect a bad printed circuit card... those are useful about half the time, the rest of the time they are entirely misleading or otherwise bogus