I have a Nest 3rd gen. The wiring for the original thermostat was a green wire going to G, white going to O/B, Yellow going to Y, and red going to R. It has been working consistently for several years. Suddenly I received a power error, and after removing the thermostat to see if there is a Common wire available, and putting the thermostat back on, it seems to continue heating after it has reached its target temp.
I have alternated from O setup to B as a test, and it runs opposite when on O, i.e. AC turns on Heat. So, I switched it back to B. It works fine on AC, but when the heat is on, it continues to heat.
I checked the voltage from the red wire to each of the other wires, and they are all around 27V. Except there is a black wire that is not being used currently. I assume this wire is the Common wire, but it only has 18V when testing from the red wire to the black. When I tried to use the black as the Common wire, it reads some error about not enough power on the RH terminal. So, at this point, it works fine except for the fact that it continues heating after it has reached its target temp.
FYI, I have turned off all of the "automatic" settings in the Nest app for learning, etc.
Please let me know if you need any further info, but at this stage, it is unusable.
Thanks,
Scott
