I am trying to upgrade our thermostat to a Wifi unit that needs a C-wire. Found the unused C wire at the thermostat end and connected it. AC wouldn't power on, went to furnace end and found it wasn't connected there either. I am not sure which of the nuts to tap into to power the C-wire and would appreciate any help. It is a Goodman unit, I found some previous posts with the same unit but the wiring on those didn't seem to match mine.

EDIT #1: Added more photos. Looked all over panels (including the inside cover) and could not find a wiring diagram. I can confirm it is a Goodman GMP050-3.

Thanks all, especially Dr. Sparks.

a bunch of wires, nutted together

same bunch of wires, different angle

distant shot of the wires connected to controller board

  • 1
    Showing a bunch of wires connected together doesn't help much - wiring colors are reasonably standardized, but that doesn't guarantee they were used on your installation. Please take pics of the wiring at the thermostat end and the actual wiring terminal block of the furnace - if there is any labeling at the wiring block, be sure it's in focus and readable. Additionally a pic of the wiring diagram label on the furnace would be very helpful. Edit these pics into your post.
    – FreeMan
    Commented Sep 27, 2021 at 12:50
  • Thanks for the response. This is an install I'm doing at my parents house, I'll get back there this afternoon and get some more pics. Unfortunately, I didn't see a wiring diagram anywhere on or near the furnace but I'll check again.
    – user142061
    Commented Sep 27, 2021 at 13:40
  • This looks like a duplicate question. The model number on the nameplate looks like a GMP050-3, wiring diagram can be found here: diy.stackexchange.com/questions/145411/… Commented Sep 27, 2021 at 13:43
  • It might be on the inside of the cover panel. I just had my HVAC replaced. The company wired it for a C wire, but I've got an old-school analog, 2-stage stat. When I turned on the heat for the first time, it blew a fuse. I reported the error code to my HVAC guy, but never even thought to look at the inside of the panel where there was a huge sticker explaining all the error codes...
    – FreeMan
    Commented Sep 27, 2021 at 13:43
  • 1
    Good find, @PlatinumGoose. I'm not sure this deserves a VtC (which you don't seem to have done) because there are no "good" answers there. Well, there is an answer that appears to be correct, but it's a bit brief, has no up votes, nor is it accepted. Might be better to get the wiring diagram (and model number) included here, then close that as a dupe of this. ;)
    – FreeMan
    Commented Sep 27, 2021 at 13:46

2 Answers 2



Tried connecting the C-wire (thin baby blue wire off the thermostat bundle) at the furnace end to a few different connections, but could not get the thermostat to turn on.

Of note, I was not putting the metal cover back on and finally realized the cover has to be on to compress a black switch in order for the unit to turn on. Success!

I'll include a picture with a blue arrow pointing to the right tap in point (picture was taken prior to adding C-wire). The Honeywell T5 turned on, and after what felt like an eternity of flashing "Cool On" the AC finally cycled on.

arrow pointing to appropriate wire bundle

  • Thanks!! +1 from me. I included your pic from the original "solved edit", since you reference it in your answer.
    – FreeMan
    Commented Sep 29, 2021 at 14:08

Ok, you'll need a voltmeter for this. Set your voltmeter to AC volts. Look on the right side of the photo. That's your control transformer. One terminal of that transformer is going to the control board to feed the R terminal. The C terminal is the opposing terminal on the transformer. Check each terminal on the transformer against the R terminal on the right. The one that measures approximately 24 VAC is your C terminal.

enter image description here

  • Thanks Dr. Sparks, I tried the connection you circled but it didn't work. I also tried the white (the only other wire coming off the budle with that red one) which is connected to the thick bluish/gray wire (which is connected to a screw on the board and I assume is a ground wire) but without success.
    – user142061
    Commented Sep 28, 2021 at 0:22
  • Updated my answer. Please try this.
    – DrSparks
    Commented Sep 28, 2021 at 0:28
  • Unfortunately I don't have a Voltmeter on hand. Anything else I could try?
    – user142061
    Commented Sep 28, 2021 at 12:02
  • Yes, just use trial and error. Try connecting your C wire to one of the secondary terminals on your transformer. See if it comes on. If it doesn't, try the other. You'll either have 0v or 24v so you can't hurt the thermostat trying this.
    – DrSparks
    Commented Sep 28, 2021 at 21:06

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