C wire not detected following factory reset

[SamIAm1]

• 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

[SamIAm1]

27.4 volts measured R-C

[JohnEE]

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.

[SamIAm1]

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.

[zoeuvre]

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 

[SamIAm1]

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. 

[SamIAm1]

 

[EdmondB]

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

[SamIAm1]

1. North America
2. Nest thermostat 
3. Lennox single-stage heat pump
4. Six wires Y-C-W-G-R-*O/B
5. Are the power breakers on and are you receiving power to your home? Yea
6. Are all the wires firmly seated into the terminals? Yes (No loose, or exposed wires)
7. Do you see any message or code that appears on the thermostat or on the app? C wire not detected
8.