I see many post with the E79 error with no solution, this will be my last effort before finally throwing in the towel on these nest thermostats. To start I have 3 nest thermostats in my house. all 3 connected to heat, then 1 connected to heat and central air-conditioning (Nest gen 3 learning thermostat). The issue is with the thermostat running the air-conditioning and heat. I was having an issue with the thermostat battery dying, I went back and forth with nest, I had someone come to house and look at the wiring. No issues found it was thought the thermostat was faulty. I bought a new thermostat which worked fine for 1.5 years then the same issue started happening again. (side note I took that "thought to be faulty thermostat" and installed it in my basement running a heat zone and it has been perfectly fine) I am back to the drawing board. what I determined through my trials is - If I only connect the heat Rh - C - W everything works fine, voltage maintains 3.8V Voc is 39v, lin is 200ma. As soon as the central air is connected Rc - G - Y the thermostat then says E79 no voltage on C wire. Unable to bypass the error I am forced to remove the C-wire which then drops the voltage voc 5v lin 20ma. and the battery in the thermostat dies within a day.
I have checked connections, making sure wires are straight and fully inserted, the voltages on the wires all test ok
Rh - c = 28v
Rc - g = 28v
I took apart the ac unit cover and checked the wiring, all look ok to my untrained eye. I called nest and went in circles on the phone,
I found this article that is identical to my issue.
https://www.googlenestcommunity.com/t5/Nest-Thermostats/Nest-thermostat-detects-C-wire-but-says-no-v...
