topic Re: C wire not detected following factory reset in Nest Thermostats

C wire not detected following factory reset

SamIAm1 — Wed, 18 Oct 2023 13:36:30 GMT

North America
Nest thermostat
Lennox single-stage heat pump
Six wires Y-C-W-G-R-*O/B
Are the power breakers on and are you receiving power to your home? Yea
Are all the wires firmly seated into the terminals? Yes (No loose, or exposed wires)
Do you see any message or code that appears on the thermostat or on the app? C wire not detected

C wire not detected following factory reset

SamIAm1 — Wed, 18 Oct 2023 00:57:46 GMT

North America
Nest thermostat
Lennox single-stage heat pump
Six wires Y-C-W-G-R-*O/B
Are the power breakers on and are you receiving power to your home? Yea
Are all the wires firmly seated into the terminals? Yes (No loose, or exposed wires)
Do you see any message or code that appears on the thermostat or on the app? C wire not detected

Re: C wire not detected following factory reset

SamIAm1 — Wed, 18 Oct 2023 13:54:06 GMT

27.4 volts measured R-C

Re: C wire not detected following factory reset

JohnEE — Wed, 18 Oct 2023 15:34:52 GMT

I had a similar issue with a later generation of the Nest. Even though I measured 27V between Rh and C, the Nest was giving me error E79 (no power to C wire). After a lot of troubleshooting, I realized the issue was that my wires were too thick (14 gauge) and were preventing the thermostat from making proper contact with the back plate. I spliced my wires to 18 gauge, used the smaller wires to terminate to the back plate and pressed them as flat to the plate as possible. That fixed the problem, the thermostat worked immediately after that.

Re: C wire not detected following factory reset

SamIAm1 — Wed, 18 Oct 2023 17:58:44 GMT

Thanks. I already have standard thermostat wire (maybe AWG20?). I just can't progress through the initial configuration. Tried 30 minutes of G Support and they just read through a list of obvious stuff so no help there.

Re: C wire not detected following factory reset

zoeuvre — Fri, 20 Oct 2023 16:46:09 GMT

Hi folks,

@SamIAm1, I'm sorry to hear that your thermostat is not detecting the C wire. Don't give up on us yet. We'd like to further check on this. Do you have the Case ID from your conversation with support? If you can provide that here, I'll look it up and see how I can continue to help.

I appreciate your input, @JohnEE.

Best,

Zoe

Re: C wire not detected following factory reset

SamIAm1 — Fri, 20 Oct 2023 20:29:57 GMT

Case 3-1831000035118

I took the thermostat to another house with a Nest, unplugged the in-service unit and plugged it in that base. I went through the configuration process and it sees the C but doesn’t recognize G.

Re: C wire not detected following factory reset

SamIAm1 — Fri, 20 Oct 2023 20:31:09 GMT

Re: C wire not detected following factory reset

EdmondB — Tue, 07 Nov 2023 01:09:53 GMT

Hey SamIAm1,

Thanks for the information and for taking additional troubleshooting steps. Just to check: when you plug the original display unit back into the base, does it still recognize the G wire?

I appreciate the help, @Zoe.

I look forward to your response.

Best,
Edmond