Complete novice here trying to install a Nest thermostat in our new home. I hooked everything up according to the Nest guide, but the thermostat gave a power/battery error message. I've gathered that it needs a C wire, but it doesn't look like one is available from the older outside unit - Goodman PGB024075-1.

Photos of the outside unit wiring and diagrams here: https://photos.app.goo.gl/CqgssCF3EULeR7YY8

Edit: I think I finally found an applicable manual - https://drive.google.com/file/d/1RLIEJSde4Kr5889qODMiiCYcrvHgTp0G/view?usp=drivesdk

I've read through a good number of similar posts, and I understand that if no C connecter is available on the board (at least I couldn't find one), I should be able to splice(?) into one of the wires coming out of the 24v side of the transformer (Pic 2). Not the red wire though. If correct, where exactly would I do this?

There are already spare small wires running into the house to the thermostat (Pic 5), so I should just need a short length of extra wire (Google says 18/5?) and to know where to connect it.

Oddly, the two capped white wires at the bottom are seemingly not connected to anything (Pic 5). They appear to run back into the unit somewhere (Pic 6). Unsure if one of those could be used?

Haven't been able to find a manual online, so hopefully the photos of the wiring diagrams inside the panels are helpful.

I'd really appreciate any additional guidance on this!

  • Do you have an economiser fitted to your unit? May 6, 2021 at 3:00
  • Hmm I'm not sure. I can take some additional photos of the outside of the unit tomorrow. Unless there is a better way to tell? Google was not too helpful
    – Mike Sharp
    May 6, 2021 at 3:56
  • Take photos of the economizer plug (it's part of the control wiring in the unit, and sits in between the Y to the compressor contactor and the Y/Y2 going to the t-stat) and post them here May 6, 2021 at 11:38
  • I think it's this? Pics 7a and 7b - photos.app.goo.gl/CqgssCF3EULeR7YY8. Tried to get two angles since the wires are bunched up against it.
    – Mike Sharp
    May 6, 2021 at 13:33
  • That's not the connector I'm after, although it may be that your unit lacks it for some reason or another... May 6, 2021 at 22:48

1 Answer 1


You'll need to do a bit of wiring work

Unfortunately, your unit doesn't expose the C terminal in a convenient place, so you'll have to do a bit of wiring to add it. You'll need a ¼" "piggyback" tab terminal suitable for 18AWG wire like the Panduit one depicted below, a 3' length of 18AWG blue TFFN wire, and a wirenut suitable for 2 18AWG wires for this.

"piggyback" tab terminal

Crimp the tab terminal onto the 18AWG TFFN, then stuff it down through the grommet in the bottom of the wiring compartment alongside the other thermostat wires. Once that's in place, slide one of the tab terminals off of the C terminal (the one with the three white-ish wires on it) of the 24V transformer in your unit and slide the "piggyback" terminal on where the terminal you removed went, then slide the terminal you removed onto the tab provided by the "piggyback" terminal. You can then wirenut the other end of that new blue wire to the wire in your thermostat cable you wish to use as a C wire.

  • Got it. Will get out to home depot and give this a shot. Thank you, I really appreciate all your help. Have a great rest of your weekend!
    – Mike Sharp
    May 9, 2021 at 21:18
  • Is it alright to use 18 awg THHN wire? I haven't been able to find anywhere that will cut only a few feet of TFFN
    – Mike Sharp
    May 10, 2021 at 21:30
  • @MikeSharp -- 18AWG THHN isn't a thing -- any building wire you get in that size will be either TFN or TFFN May 10, 2021 at 23:10

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