Please help.

[briandgoldy]

We checked and checked and checked and there is nothing wrong with our Common C wire. But we continue to get the E79 message and a low battery message. 

Also, when I tried to add the Learning thermostat to my Nest App or Google Home, it asked me to scan a QR code, which didn't work. Then I tried to enter a code, but the only 4 alphanumeric that came up was NOT the serial number of my device.

WHAT IS GOING ON?!?!

[zoeuvre]

Hi briandgoldy, 

Thanks for posting here in the Community forum, and I'm sorry to hear about the situation. I want to check if you are still in need of any help. If so, E79 code means no power to C wire detected. If you have checked the C wire and there's nothing wrong with it, I recommend you also check for its voltage using a voltmeter or a multimeter and following the steps 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 RC/RH wire.
2. Place the black lead on the C wire.
3. The normal readout should be somewhere between 20 and 30 V (24 V is most common).
4. Suggestions to keep in mind if you get a very low readout or nothing at all:
   • Test one more time.
   • Verify settings on the voltmeter are correct.
   • Perform the test properly.
   • Verify leads are plugged in properly.

Also, if scanning the QR code does not work, select continue without scanning instead and enter the Entry Key of your Nest Learning Thermostat that is found on its Settings > Nest app or check the back of your thermostat display; the six-digit alphanumeric is the Entry Key.

Let us know how it goes.

Best,

Zoe