I have a very similar problem to the question Why is Thermostat Y wire connected to C terminal on furnace? . The big difference is that I dont have an "extra wire" to use. Only 4 wires go to my thermostat. This is the situation:

  • Thermostat: I have 4 wires connected to G, W, R, Y.Thermostat wires

-Furnace: The wires coming from the thermostat are connected to G, W, R and C Furnace wires

  • Between thermostat and furnace: There is a connection in the blue cable. From what I´ve read this sends C and Y to the external compressor. And this makes sense in my case because that cable goes to the outside compressor.

External compressor?

I´m trying to install a smart Honeywell T9, and I need a C wire. I also bought the C wire adapter, because I saw that I had a Y in the thermostat (unfortunatelly I didn´t check the furnace before buying it) Is there any way that I can connect this? or Do I need to run a new wire just for C?

Thank you very much for your help.

  • Before you do anything (if you haven't already) peel back a bit more of the brown sheath and check to see if there isn't a fifth wire cut back and hiding in there.
    – gnicko
    Jul 12, 2023 at 22:46
  • Thanks gnicko, I have, but there's not a 5th wire. If there's no better option I can run a new wire. If so, where would I plug it? Together with the other 3 in the blue cap? Jul 13, 2023 at 0:54
  • Can't really tell. There's going to be a way to connect it to power from the circuit board, but I don't see where that is in the picture. Got more pictures?
    – gnicko
    Jul 13, 2023 at 1:15


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