I noticed one of my Nests was offline. It did not have a common wire. I charged the Nest using a USB and used an unused wire going to the thermostat to provide the common. The red wire on the transformer connects to the red going to the thermostat, so I attached the terminal with the white wire on the transformer to the unused wire going to the Nest. I then connected that to the Nest C wire. The next gave me an error A79, indicating a C wire, but it was not connected to anything. I checked the transformer, and it was producing 26 volts. I went to the thermostat, and there were 26 volts between the Rh and C wires. I checked the C wire, stripped 3/8 of an inch, and seated it in the thermostat hole. After several tries at reseating the C wire and getting the A79 error, I tried a second nest thermostat on the base, and it got the same error. I can only figure out that the base may be defective. Any ideas? I have nine Nests installed so I may try swapping bases to verify the problem before buying a new base.
