Confused by the wiring, hope someone can help me out. At the furnace there's only four wires in the cable (R, B, G, and W), but at the other end, a yellow wire appears. It's not reflected in the picture of the thermostat wiring, but the blue wire is there, behind the plate. I figured it was the C wire and connected it to the Ecobee, but that didn't work. Opening up the furnace, I can see why, but I'm not sure how it should be wired. I understand that the diagram attached makes it pretty easy, but the previous thermostat wiring doesn't make any sense with the wiring at the furnace. For instance, the green wire isn't attached to anything at all! The blue wire is attached to the furnace, but unless it's tied to another color wire on the way to the thermostat, it's also not connected to anything. I have zero idea what the yellow wire is attached to. Do I just need to get a multimeter and test each wire??

Any help is appreciated!

Lennox model: CX34-43C-6F-1

Furnace Wiring1 Furnace Wiring2 Old Thermostat Humidifier1 Humidifier2 Wiring Diagram


1 Answer 1


The issue was that the C and Y wires apparently went nowhere. I traced them as best I could, but couldn't figure it out. Fortunately, there was a deprecated humidifier control box right next to the thermostat with two wires. It was cut close enough to the furnace that I could remove some staples, stretch it, and get it to the control panel. I used those wires as the C and Y wires and everything is fine.

In the future I will need to figure out how to wire up the Aprilaire. I don't want the humidifier at this moment, but I mixed up the current configuration in order to get the thermostat working.

  • 1
    Sounds like a bit of a bodge, but if it works... It would be nice of you to include a note somewhere, indicating what you did for the next person who has to work on this. Sure, you expect to be the next person, but, life may get in the way and you may not be. Or, it may take long enough to get back to it that you've forgotten what you did...
    – FreeMan
    Commented May 4, 2022 at 12:58
  • That's a great idea, @FreeMan . When I finalize it, I'll add a note/labels to the wires to make it extra clear. Even if I am the next person, with my memory, I'll definitely curse my past self if I don't lol. Commented May 13, 2022 at 12:59

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