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E79 Error - Common wire installed worked for a year

Community Member

We recently had the air ducts cleaned in our home due to some construction. They turned off the power to the furnace and when we went to turn it back on, the nest is saying that there is no common wire. I installed our Nest a year and a half ago with a common wire and had no issues. 

I'm currently able to bypass the error message so we can control the thermostat however we are unable to make any adjustments in the Nest app. 

The Nest did lose power because we turned off the power to the unit.


Community Specialist
Community Specialist

Hi bjbillinger, 


Apologies for the delayed response but thanks for reaching out to the Google Nest Community. I want to check on this further. 


A few questions — what type of Nest thermostat is this? Have you ensured that all the wires especially the C wire are properly seated? Also, could you ensure that  the power to your system is on? The power switch will usually be in your breaker box or fuse box, but in some homes it will look like a light switch near the heating or cooling equipment. 


I also suggest that you try the suggestions on this link when checking on your thermostat wires. 


Keep me posted.