topic Re: C wire suddenly not detected in Nest Thermostats

C wire suddenly not detected

Stang233 — Mon, 02 Oct 2023 03:32:44 GMT

Pulled my nest off to change the batteries. It’s a 3 wire with the C from a transformer. I have 26v from C to R. Nest will not recognize C wire now.

I have undone the wiring and reconnected. Factory reset the nest. It will still not detect the C wire. Now I cannot get into the nest for setup to reset again. It has worked fine for 23 months.

Re: C wire suddenly not detected

zoeuvre — Thu, 05 Oct 2023 16:17:47 GMT

Hi Stang233.

Oh no! Sorry to hear about that — let me take a look at this for you. A few questions:

Are you located in North America or Europe?
What type of Nest Thermostat do you have?
What is the make and model of your heating, ventilation, and air conditioning (HVAC) equipment?
How many wires are there?
How are they labelled?
Are the power breakers on and are you receiving power to your home?
Are all the wires firmly seated into the terminals? (No loose, or exposed wires)
Do you see any message or code that appears on the thermostat or on the app?

Also, send us a photo of how the thermostat wires are connected to your thermostat base, and please take note that Nest thermostats can only receive 24 V power from your HVAC system. If your thermostat reads beyond that voltage, that will result in overcurrent.

I'll look forward to your response.

Best,

Zoe

Re: C wire suddenly not detected

Jdaniels — Mon, 09 Oct 2023 23:28:13 GMT

I too had this same issue. Except mine went out after a thunderstorm which caused house to lose power. When power came back on it said c wire not detected I tried to reset and it no longer is connected. I have 3 nest systems in my house. Upstairs main floor in basement 2/3 work fine but the kitchen won’t let me do anything and I cannot turn the heat on.

Re: C wire suddenly not detected

zoeuvre — Mon, 09 Oct 2023 23:40:09 GMT

Hi Jdaniels,

Yikes! Let me check on this for you. Could you also answer the following questions I asked above so I can take a closer look? If you have also tried any other troubleshooting steps, please let me know.

Thanks,

Zoe