I have 5 wires running to the thermostat from the furnace. The blue wire is not connected on either side. I'd like to connect it as my c wire for a smart thermostat.

On the furnace I see W R Y G and what looks like P on the top right. So no C. Am I out of luck or can I connect the C wire elsewhere?

Make and model of furnace: Trane electronic ignition xe 70

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  • What make and model is your furnace? Sep 2, 2017 at 18:54
  • Trane electronic ignition xe 70
    – MahoreLee
    Sep 2, 2017 at 18:58
  • Can you post a picture of the wiring diagram label on the furnace? Sep 2, 2017 at 19:14
  • I feel like I've checked everywhere and can not find it. Back of every panel, in the furnace, around it.
    – MahoreLee
    Sep 2, 2017 at 19:24
  • Strange -- does the transformer have a nameplate label on it at least? Sep 2, 2017 at 19:27

1 Answer 1


The other white wire is your clue

See that two-wire cable with the red wire connected to the yellow (Y) wire from the thermostat cable? That cable controls your compressor, and the white wire from it must connect to the equivalent of the C terminal on your air handler for your air conditioner to work. So, connect whatever wire you're going to use as C to the same terminal as that white wire, and you'll be good!

  • 1
    To be sure: follow the white wire from the black sheath that has just the red and white wire in it, NOT the white wire from the thermostat. Correct?
    – MahoreLee
    Sep 2, 2017 at 20:14
  • @MahoreLee -- correct! Sep 2, 2017 at 20:55

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