E79 error after power outage

[ExRagna]

Had the Nest Thermostat 3rd Gen for over 2 years. Has worked fine up until a big storm hit my area and lost power for 8 hours. Come how saying no power to C wire/error 79. Checked the C wire and it gets 27-28 volts. 

[zoeuvre]

Hi ExRagna, 

Thanks for posting here in the Community forum, and I'm sorry to hear about the situation. Let me take a look at this for you.

The E79 code means no power to the C wire was detected. In this case, the thermostat detects a wire in the C terminal but can't complete a power-sharing circuit. Please collect the power readings in Settings > Technical Info > Power, then list down the following:

• Battery
• VOC
• VIN
• lIN

I'll look forward to your response. 

Best,

Zoe

[zoeuvre]

Hi there,

I want to check and see if you are still in need of any help. Please let me know, as I would be happy to assist and answer any questions you may have.
 

Thanks,

Zoe

[ExRagna]

• Battery = 3.76V
• VOC = 0.04V
• VIN = 0.04V
• lIN = 2ma

Pretty convinced the actual base is blown. I went to test my thermostat at a friends house who also has same thermostat. Switched out base and thermostats. My base does not work at his house and switched back to their unit and worked fine. Took their base to my house, worked as normal. Switched back to my base, same error. 

[zoeuvre]

Hello there, 

Thanks for getting back to us. May I ask how you checked the voltage of your Nest Learning Thermostat's C-wire? Did you use a voltmeter or a multimeter to check for it? If so, did you do the same as below?

1. Set the dial to 200 VAC.
   • If the multimeter doesn’t have a 200 setting, choose 100 or 250 VAC.
   • The general rule of thumb is don’t set it too high (500+) or too low (50).Place the red lead on the R wire.
2. Place the black lead on the other wire being tested (W1, Y1 or C).
3. The normal readout should be somewhere between 20 and 30 V (24 V is most common).

Also, please try to remove and then reconnect the C-wire by first turning off the power breaker before reseating it. If the voltage still shows the same, turn off the power breaker again, remove the C-wire, and then let the thermostat work without it. Check the power readings again and identify if they have changed significantly without it.

Regards,

Zoe

[ExRagna]

Yes I used a multimeter and the C wire was reading 27 - 28V. 

Edit: If I don't plug in the C wire, it starts to act like everything is working normal. But thermostat itself will die due to not charging. 

[zoeuvre]

Hi there ExRagna, 

Thanks for the information. Don't give up on us yet. We'd like to further check on this. Could you fill out this form and let me know once you're done? 
 

Kind regards,

Zoe

[ExRagna]

I have completed the form. 

[zoeuvre]

Hey ExRagna, 

Thanks for the update. We got the form you've submitted — thanks for filling that out. I'll consider this post complete and lock the thread within 24 hours. Please keep an eye on your email, as someone from our team will reach out to you to assist you further.

All the best,

Zoe

[ExRagna]

okay thank you. 

[Jenelyn_O]

Hi there,

No worries, please feel free to create a new thread if you have any questions or concerns, as we'll be willing to assist you from there. We'll be locking this thread after 24 hours.

I appreciate the help, Zoe.

Cheers,

Jenelyn