e79 Error

[justs]

I have read all of the e79 error threads here and a couple dozen others around the web with no luck. 
I have two Nest thermostats and suddenly 3 days ago the one upstairs showed an e79 error message, no power to C wire. 

Nothing changed so I don't know why this would occur. I have tried everything I can think of. Exchanged the two Nests, same error on each when connected upstairs. I swapped the two wiring harnesses and again whichever one was upstairs yields the e79 error.

I stripped all the wires cleanly and installed again, still no change.

I flipped the main house breaker and that worked. No c wire error, for the evening. Next a.m. error was back.

I've checked the wiring at the cooler upstairs, the furnace in the basement, the cooler outside, no apparent issues and no fix.

I walked through all of this with support and reset the thermostat, no change. 

Anything I have missed?

[MelbaDT]

Hey justs, 

Thanks for reaching out and for providing a detailed description of your concern. 

Based on what you've already tried, you need to have your system checked by a professional. Please contact an HVAC technician or a local pro for more help.

Best, 

Melba

[MelbaDT]

Hey there, 

How's it going with your thermostat? Have you reached out to a technician already?

Best, 

Melba

[Ryan_G]

Hey there,

Thanks for being here in the Google Nest Community. 

I'd like to bump in here and make sure that you’ve seen our response.
Please let us know if you have any other questions or concerns as I will be locking this thread in 24 hours.

Have a great day.

Best regards,
Ryan