Nest Therm Gen 3 E79 Error code after a power failure

[Jeis123]

After working fine for three years, my Nest Thermostat is showing an E79 Error after a power failure. I can still communicate with the thermostat, but I can not see in the NEST Android App settings on how to restart the unit remotely. Can this be done? If so, where do I find the restart option?

[moralesvicente]

Hello Jeis123, 

 Thank you so much for posting in this community. I’m sorry to hear that your Nest Learning Thermostat (3rd gen) is showing an error code E79 after a power outage. No worries, it’s a pleasure to assist you. 

This error code (E79) means that the thermostat detects a wire in the C terminal but can't complete a power-sharing circuit. So, let’s perform these recommendations.

• Check the system power: Before troubleshooting your thermostat, confirm your system has power. Locate the circuit breaker or fuse box, or a dedicated switch near your heating/cooling unit. Ensure the correct switch(es) are on, then proceed to check your thermostat's functionality.
• Verify the thermostat wires: it’s important for your safety, and to prevent damage, always disconnect power to your heating/cooling system and thermostat at the circuit breaker or fuse box before working with any wiring.
  1. Pull off the thermostat display.
  2. Check that the wires are inserted correctly in the thermostat.
  3. Check that each wire is fully inserted into its connector and that every connector with a wire is down.
  4. Check that each wire has 6 mm (⅜") of exposed wire.
  5. Turn the power to your system back on and check to see if your thermostat is working.
  6. Check that each wire is connected properly to the system board.

Let me know how it goes; I'll look forward to your response.

Best regards, 

Daniel