Google Nest Thermostat: E77 Error

[Cwv1jz]

I am installing a new Nest Thermostat but am running into some troubles; specifically the e77 error. 

The install was going smoothly and during the initial test the fan and heater both turned on. I have no air conditioning system. However after the successful testing, I received the E77 error saying I had a wiring issue. 

My original thermostat was wired as follows; 

Blue wire : B 

Green wire : G 

White wire: W 

Red wire: R (bridged to RC) 

When installing the Nest, I was instructed to wire as follows: 

Blue wire: O/B 

Green wire: G

White wire: W1 

Red wire: Rh

Any ideas? 

[Eitanros]

Hello @Cwv1jz,

Thank you for reaching out to the community.  I'm sorry to hear you're receiving the E77 error code on the new Nest thermostat. I'm happy to help you!  Let's work together to fix the error code. 

Since you do not have a Heat the Pump system and the blue wire is connected to the "O/B" terminal on the Nest thermostat and previously was connected to the "B" terminal. 

Please follow the next steps: 

• Turn off the HVAC breakers. 
• Remove the blue wire from the "O/B" terminal. 
• Plug the wire into the "C" terminal.
• Turn back on the HVAC breakers. 

Please let me know how it goes.

Best regards,

Eitan

[Cwv1jz]

Thank you for getting back to me! After moving the wire from the O/B port to the C port I am receiving the following error: 

E74 - No power to Rh detected. 

Any suggestions? 

[Eitanros]

Hi @Cwv1jz, 

Thanks for the follow-up information.

Let's try the steps in this guide:

• Wires must be firmly connected to the terminals. Ensure at least ¼ to ½ inch of the copper wire is exposed, straight, and inserted fully into the correct connectors. If needed, strip and reinsert the wires. If necessary, Strip and re-insert the wires
• Wires that are too thin (smaller than 18 gauge) won't work with the Nest Thermostat. A Nest Pro or local technician can replace them if necessary.
• The Nest Thermostat needs 18 to 22 gauge solid-core copper wires. Stranded wires are incompatible. If you have stranded wires, a Nest Pro or local technician can replace them.
• Note: If you feel comfortable, please check the voltage of the RH wire using a voltmeter. The normal readout should be somewhere between 20 and 30 V (24 V is most common).

Please let me know how it goes.

Best regards,

Eitan

[Cwv1jz]

Hi Eitan, 

Unfortunately I don’t have a multimeter to check the voltages. I can get one tomorrow to test that. 

I can confirm the wires are 18 gauge solid copper wires and are connected securely. I tried re stripping/connecting them and am now getting the following error: 

E79 - No power to common (C) wire detected. 

Any further suggestions? 

[Eitanros]

Hi @Cwv1jz, 

Thank you so much for the information provided and the steps you've tried to take regarding the error codes you're receiving. 

 The team would like to learn more about this behavior. When you get a chance, please fill out this form and let us know when you’re finished. We’ll have someone reach out to you via email from there.

Feel free to reply if you have further questions.

Best regards, 

Eitan