We have had a longstanding problem with our Gen 3 Nest thermostat at our cottage where we would randomly get a E79 no “c” wire error despite there is a “c” wire perfectly connected and the wiring verified numerous times. We eventually suspected without solid evidence, not being present when it happens, the problem coincides with power outages (we get notifications from the hydro company) that may flash on and off a couple times. We have the same thermostat at our home which didn’t seem to be afflicted the same way until NOW where we were present during a short 15 minute power outage and sure enough up came error E79 no “c”wire.
it is now clear that the Nest thermostat is the issue as we have the same problem at two different premises during short power outages. As this knocks the furnace out of order and can only be fixed on location by turning the power to the furnace off and on, which is impossible if we’re not at that address, baring a satisfactory solution we will be forced to take out both thermostats and replace with another brand and likely switch our entire home eco-system away from Google.
