I just installed a nest thermostat. The previous thermostat did not have a C wire attached. It G, RC, (RH Jumper), Y and W. When I hooked up the nest thermostat it was short cycling the blower fan until I pulled the AC (Y) wire.

I've been reading this happens if there is no C wire then the nest is trying to draw power from the furnace.

I then tried to attach the blue wire which was unused in the previous thermostat to the nest C terminal. The nest threw an error saying it was not getting power from this wire.

This isn't an ideal situation because once summer comes i cannot use my AC. I'm looking at my furnace and looks like the C terminal is connected but that wire is also joined on the Y terminal? I attached a picture of the furnace wire configuration.

Wondering if there's something I can do here to give the blue C wire power at the thermostat or if I should go with the venstar add-a-wire?


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Old thermostat wiring:

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  • Did you read this post before posting: diy.stackexchange.com/q/169494/97780
    – Solar Mike
    Commented Dec 14, 2019 at 18:41
  • Yes I think mine is a little different. It looks as if the C wire is connected to the Y terminal in the furnace but not the C.
    – jrowe08
    Commented Dec 14, 2019 at 19:00

1 Answer 1


What you need to do is to move the Blue wire at the furnace over to the "C" terminal at the furnace. Then at the new thermostat connect the Blue wire to the thermostat's "C" terminal.

Note that at the furnace the White wire currently on the "C" terminal goes into a separate cable bundle that is paired with the Red wire in that same bundle. That cable most likely runs off to the air conditioning compressor unit.

It is not clear what the Yellow wire in the cable bundle from the thermostat connects to at the furnace end. There is a good chance that this is connected incorrectly or not at all. My understanding is that this Yellow wire should be connected into the "Y" terminal at the furnace where the miss-wired blue wire used to be.

  • The AC runs correctly with the old thermostat. If it was wired incorrectly or not at all would that have been the case?
    – jrowe08
    Commented Dec 14, 2019 at 20:34
  • 1
    Like I said it was not possible, from your pictures, to see how the Yellow wire from the previous setup was connected at the furnace. Research that and it will help to clarify.
    – Michael Karas
    Commented Dec 14, 2019 at 20:37
  • Thanks for this. The blue wire at the furnace was connected to the yellow wire in between the thermostat and the furnace. The blue wire from the thermostat was unused. I ran a new wire from the unused blue to the blue terminal in the furnace and everything's working!
    – jrowe08
    Commented Dec 17, 2019 at 1:55

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