I'm about to go out and buy a smart thermostat for my place and had a question about the 'c' wire (common wire). From what I've read online, the blue wire is commonly known to be the c wire for the thermostat. Below is a picture of my current thermostat terminal and I am somewhat confused because my blue wire is set into the 'Y' terminal and the yellow wire is unused. Does anyone know if the blue wire here is really the c wire? thermostat terminal

EDIT: I got into the furnace motherboard to look at the terminal and here is what I have found:

(yellow wire is not pictured below but it is clearly not connected to the terminal.)

enter image description here

  • 1
    Colors on furnace wiring are advisory. Nothing is hooked up to your C terminal. Might want to see if any wires are hooked up to the 'C' wire (or the neutral side of the 24v transformer). Worst case you can use an add-a-wire.
    – gbronner
    Sep 19, 2022 at 20:03
  • Yes, where does the yellow wire lead at the air handler end? Sep 20, 2022 at 2:46
  • Unusually, you have 2 red wires, one of which is connected to C at the furnace. Confirm that the red wire on R at the furnace connects to RH at the thermostat, then figure out where the red C wire at the furnace goes.
    – FreeMan
    Sep 20, 2022 at 14:31

2 Answers 2


The blue wire connected to the "Y" terminal is strange. The yellow wires usually control the compressor. The yellow wire could have been damaged in the air handler so the installer used the blue. You'll have to follow that wire into the air handler and see how it's hooked up in there. There should be a wiring diagram attached in the air handler

  • Now that we have a picture of the wiring at the furnace end, the blue wire on the "Y" terminal of the thermostat isn't strange at all. The blue wire is also connected to the "Y" terminal on the furnace! Up until that extra pic was added (after this answer), it was unusual...
    – FreeMan
    Sep 20, 2022 at 14:29

That 2nd picture helps clear things up, although it doesn't explain why blue was used instead of yellow. If it's good, you have 2 options:

  1. Simply add yellow wire at furnace to C. If you do this, I would use the labels that come with the new thermostat to identify your wires for future reference.

  2. Move blue wire at furnace to C, and add yellow wire to Y. Then at the thermostat side your blue and yellow wires will match the terminal labels.

(If that yellow wire is broken, things get a bit more complicated.)

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