Can't get new thermostat to detect yellow wire

New thermostat installation, old didn’t have C wire but I did have a wire wrapped around the others at the control board, so I used that. Everything looks good to me, but it’s obviously not, as no yellow wire detected, no A/C. Help?

Do you have an image of how your furnace and thermostat used to be wired?

Also, do you have an indoor unit and and outdoor unit, or just one combined unit outdoors?

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There’s an indoor furnace and an outdoor a/c compressor.

The blue Rc wire wasn’t connected at the other end originally

The blue wire at the thermostat is likely connected to the transformer in the air conditioner. Do you have 2 sets of wires in your wall behind your thermostat?

Also, on the furnace board, it looks like green used to be on G , yellow used to be on T, red used to be on R, and white used to be on W, but when you re-wired it you connected white to W, red to R, yellow to Y, and green to T.

Now, T is the same as C, so without a C wire, you would need something between there and the thermostat, but the lack of a Y wire at the furnace in the original wiring isn’t great. Does that yellow wire at the furnace enter the same wire bundle as the rest of the wires? You might need to open up your air conditioner and take some pictures of the wiring there for me to be able to help you.

You might have a series wired air conditioner, if you don’t have 2 sets of wires in the wall behind your thermostat.


No, only one wire “bundle” with 5 wires in the sheath running from the thermostat to the furnace.

In reality, I think it’s one wire bundle to the thermostat, but the hole thru the paneling is so small, and there’s so little slack, that I can’t determine for sure. But originally I had no Y and no Rc connection at the furnace, the G wire was connected at both ends, and the wire from the Y terminal on the thermostat went to the T/C connection at the furnace. I’m lost…

I think you have something like this going on:

This will prevent you from using the C adapter at the furnace, but you might be able to use it at the A/C unit outside. I’m hoping that all the wires go from the furnace, to the A/C unit, and then to the thermostat.
Can you turn off your disconnect to your air conditioner and take a picture of the inside with the wiring panel off?

Are these any help? It’s kinda messy out there… :slight_smile:

Hmm. That’s not what I was hoping for. You have a splice somewhere between the furnace and the air conditioner.
Can you follow the wire after it leaves the furnace ? It should split up into multiple wires somewhere. If you find that spot, can you take a picture?

Yeah, that might be the case…

Oh, and thanks so much for taking the time to help me with this!

ok, this is what you have now:

Easiest way to fix this is to do the following:
0, Turn off the power to the furnace and leave it off until I tell you to turn it back on (Turning it on too early will probably blow something up, so don’t do that.)

  1. take the blueish wire out of this wirenut
    then add it to this wire nut

  2. Back at the furnace, connect the wires back the way they were originally.

  3. At the wyze thermostat, connect the red wire to Rc, The blue wire to C, the yellow wire to Y1, the green wire to G, and the white wire to W1.
    Then you can turn the power back on.

Reset the thermostat and tell it that your “old thermostat” has the following wires:
Rc, C, W, G, Y

By doing this, your schematic would look something like this:

At this point, I have to assume you’re a genius.

Seriously, thanks so much for your help with this - this elegant solution worked perfectly. I’m almost ready to tackle the second system! :joy:

@speadie I appreciate the time you took to figure this out for me!

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