When I got the "low battery" warning on my Nest 1.4, I replaced the batteries several times, with fresh ones, and, today, put in ones I had just purchased. Still got the warning. So I checked the wiring, which doesn't require an electrical degree. Pulling off the face, I saw that the blue wire was properly connected to the tab with the "C," for "common," or 24 volt AC power wire. (The common wire should supply power to the thermostat, so that it doesn't even need batteries.) Then I went to the furnace, unscrewed what held the doors over the internal parts, and saw that the blue wire from the thermostat, was properly secured. Below the block of screws for the wires from the thermostat, was a fuse (with an "E") that I pulled out between two fingers; there was no black, burned look, so I plugged it back in. Voila! After putting the door back on the furnace (there's a switch that requires the door be back in place), I no longer have the low battery message!!!! (I'm not even sure I need the batteries.) Why can't Nest tell us this?
