Good morning. I am looking to hook up a Nest 3rd Gen thermostat that I purchased. I noticed I had a C wire upon pulling the old thermostat. Problem is it is not connected inside of the furnace. I have a Goodman 60k furnance. Model-gmpn060-3 . I originally connected without the C wire and had cycling issues and immediately removed it. Looking for help. Thanksenter image description here

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Here are a few photos i took. Just need to know where to hook the blue wire up inside of the furnace. Also as you can see the blue wire coming out of the brown sleeve is not hooked to anything inside of the furnace

Theres also some wires hooked up outside of my furnace on the side of it enter image description here

  • Can you post a photo of the actual wiring inside the furnace? Jul 20, 2019 at 13:34
  • A picture inside the furnace would help us be more specific, but generally you need to attach it to the opposite side of the 24v transformer than the hot line. In the last wiring diagram, that seems to indicate there are several black wires coming off of it, one running to the number 6, one to ground, and one to the component labeled GV.
    – Matt
    Jul 20, 2019 at 16:30
  • I will take a photo as soon as i can. Its super hot today and i dont want to turn off the air. Lol Jul 20, 2019 at 18:55
  • Alright guys can you see the new photos Jul 28, 2019 at 2:25
  • Anyone have input? Aug 2, 2019 at 10:56

2 Answers 2


Add it to the wirenut with the fat blue wire in it

On a typical split-system with air conditioning, there will be a two-wire cable running out to the air conditioner with one wire connected to the yellow Y wire and the other wire connected to C; this cable carries the call-for-cooling signal from the indoor unit to the compressor unit's contactor.

As a result of this, we start our trace at the Y wire, and trace the wire connected to it back to the outdoor unit's control cable. From there, we can deduce that the other wire must be connected to C in order for the air conditioning to work, so we can simply add the blue C wire going to the thermostat to that wirenut. This is confirmed by the fact the wiring diagram you posted shows that connections to the secondary-side ground (C) are all blue wires, and the wire nut in question has a fat blue wire going into it.


In the third picture, your transformer is in the top left corner. The side with red and blue wires coming out should be the 24v side, check with a multimeter to confirm. Follow those blue wires and maybe there's a terminal in the control board you can use, or a wire nut you can twist the c wire coming from your thermostat into.

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