Right now I don't have specifics on my central air unit (make, model, etc) but hopefully the info I provide will help.
I have a Second Generation (and Third Generation) Nest Thermostat hooked up to the Central Air Unit. I receive intermittent messages of "not enough power" being sent to the thermostat. (The reason I mentioned both Nests is that I've tried both units).
The wire contains one hot wire (Rh), one for the fan (G), and one for the cool/compressor (Y1).
I used a multimeter to test the voltage, and when I touch the positive probe (red wire) to the Rh wire and the negative probe to the G1 wire, I get a strong enough reading (between 27-28 volts). But when I do the same to the Y1 wire I get next to nothing.
After some research, I found people recommending changing the fuse in the unit itself... but when I lifted the panel, I didn't see any fuses that could be replaced. Moreover, the power drain doesn't seem to be consistent (i.e. today it is showing a poor voltage reading and tomorrow it will be fine).
I don't believe its the Nest because I've tried different ones and get the same results. Sorry for the long question, but I wanted to be specific. Any ideas on what is causing the power generation problem?