Trying to install an Ecobee 3 Lite thermostat. The old thermostat had a W, G, R, and Y wire connected, but no C cable which is needed.

old thermostat

However, I saw an unused red and blue wire in the wall and pulled them out.

new wiring

I realized that my control board only seems to have W, G, and R labeled, and instead of using screws it uses quick disconnects and prongs.

control board labels

Those cables are wire-nut to the cables coming from the actual thermostat. You can see that there is also the unconnected blue cable and the red cable wire-nut with another red cable coming from the outdoor fan unit I believe, and then runs into the board. There is also the yellow cable from the thermostat wire-nut with a white cable also coming from the outdoor unit (again, just a guess).


The two red wires that are wire-nut together with the thicker blue-ish cable seems to connect to behind the transformer?

thick blue cable to transformer

Could I somehow use the un-used red wire at the thermostat as a C wire? Do I connect a cable to the unused blue wire from the thermostat into my transformer somehow? Any tips or suggestions would be appreciated.

I have the PEK extender (uses C and R from the board as one R cable to the thermostat) that came with the new thermostat and initially thought I would need it but finding the unused cables made me think otherwise.

ALSO: there is an unused 24V labeled screw slot above the W, G, and R prongs that you can see in the pictures. Could this be used as a place for the C cable?

EDIT: Here is the electrical wiring diagram wiring diagram

  • Lots of good info here to help us help you, but there's one vital piece of info missing. What make/model is the furnace? Additionally, a picture of the wiring diagram (if there is one), likely on the inside of the cover, would be most helpful. Finally, make sure you do something about the insulation on the white wire at the stat end. Strip off the torn pieces, then cut the bare wire to the proper length before installing it in the new stat.
    – FreeMan
    Sep 30, 2021 at 18:32
  • @FreeMan Ah forgot about that. Just added that to the post. Added some electrical tape to the white wire on the thermostat. Thanks!
    – Dan
    Sep 30, 2021 at 18:44
  • @Dan, I've correctly answered your post below, let me know if its somehow unclear. Thanks!
    – JPhi1618
    Sep 30, 2021 at 20:03
  • @Dan Please merge your accounts so you have access to edit, comment, and accept answers on your own question. Posting multiple responses as answers can result in an automated lock that can prevent others from assisting you.
    – BMitch
    Oct 1, 2021 at 13:23

1 Answer 1


You see that green and blue wire that are connected to the chassis of the air handler in the top corner of the this picture? That is where the C wire needs to connect. You can either place it under that screw, or you can see if the green or blue wire end with a wire nut that you could add another wire.

Edit: Actually, it looks like the brown wire for the thermostat is already connected to the thick blue wire in this picture. So that brown wire should be ready to use. Check with a voltmeter at the thermostat to verify that you have 24vac between the red and brown wire.

  • Tried your method. The thermostat got all set up and working. Around 10 minutes later I realized it was not responding on my phone. Went to go check it out and it was off. Could it be possible that it got shorted? Should I take it off and check the voltage of the C wire again?
    – Dan
    Sep 30, 2021 at 20:32
  • Yes, check the voltage. Based on the way its wired, that thick blue wire is connected to the brown wire and one of the wires going to the outside unit. That is the "common" wire for sure. The other thing you can do is swap the brown and blue wires. It's possible the brown wire is damaged somewhere along the way. I'm 100% confident that is your C wire connection tho.
    – JPhi1618
    Oct 1, 2021 at 14:27
  • Thanks for all your help. Apparently, the issue was that the door to the furnace control board got loose causing it to shut off. Putting it back in correctly and installing a few more screws kept it secure and now all is well!
    – Dan
    Oct 2, 2021 at 22:29

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