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My Nest thermostat has been running fine for quite a while, but recently it started complaining about low battery and wi-fi gets shut down due to power issues. I've been looking for the answer, and I found out that my Nest is not connected to the C-wire. Weird thing is that my Lennox furnace control panel has C-wire connected to blue wire, but Nest does not seem to recognize the C-wire (it says power is not detected to C-wire). Right now, I disconnected C-wire from Nest back panel, because it complained that C-wire does not have power. Can you please tell me what could be wrong? I am posting the pictures below (click to enlarge them). Any help is appreciated!

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So you have two C terminals on the control board? I see a blue wire on one of the C terminals and then a red wire on the C terminal next to it. The red wire goes to the outside unit to make it turn on and off, so that one should have power for sure. If you don't have a multimeter, you can just move the blue wire to be with the red on the other C terminal.

If you want to troubleshoot a bit more, you can use the continuity testing mode to make sure the two C terminals are connected. Also check the AC voltage between the R and C terminals to make sure they both have around 24v. Also check the wires at the thermostat to make sure there is 24v between R and C. There is a chance that the C wire circuit on the Nest has died, so verifying voltage at the board and at the thermostat is your best bet.

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  • Thanks so much for your help! I tried with your suggestions and here’s what I found: I confirmed that R and C terminal in the furnace control board has 24v. I tried blue and other two spare wires connected to both C terminals but weirdly none of them has 24v linkage at the nest thermostat end lines with Rc line. I also checked Rc and G or Y lines and they have good 24V. That’s where I stuck. The only explanation is that blue and two other spare wires are cut somehow in the middle. Is there any other suggestions you can provide? Thanks in advance!
    – scifi88
    Dec 8, 2021 at 20:14

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