E79 No power to C wire

[CEV]

We got a new furnace a couple of years ago. I had a nest learning thermostat before that. The installers put in a regular thermostat. I reconnected the nest thermostat today after it had been sitting in a drawer for a couple of years. Now I'm getting this E79 error. Wires are pushed all the way in. Regular thermostat works, rye nest does not. Suggestions?

[CEV]

I reconnected the other thermostat and it's working. So why is the nest thermostat not working and showing that error?

[Markjosephp]

Hello CEV,

Thank you for taking the time to let us know about your inquiry.

It sounds like you've encountered an E79 error after reconnecting your Nest Thermostat following a new furnace installation. First off, I appreciate you troubleshooting by ensuring the wires are securely pushed in. Since the regular thermostat works fine, let's focus on resolving the Nest Thermostat issue.

Technically, the thermostat detects a wire in the C terminal but can't complete a power-sharing circuit. You can try removing the C wire and leaving it disconnected. Then, ensure that the remaining wires are still properly connected before plugging the display back into its base. Restart to refresh the system. Lastly, gather the power readings in the Technical Info with and without the C wire installed.

Let me know how it goes.

Best regards,
Mark

[CEV]

I will give that a try this weekend and see if that works. But from what I understand about the C wire is that it gives power to the thermostat. If I do not connect the C wire to the thermostat, then I would need to take the display off periodically to charge it?

[Markjosephp]

Hi there,

Great! Giving it a try this weekend is a good plan. You're correct that the C wire provides a constant power supply to the thermostat. Without it, some thermostats, like the Nest, may rely on a battery, and you might have to charge it occasionally. To verify the issue is resolved, check if the wiring code no longer shows on the display. If it does, the C wire may not be properly grounded and needs to be energized or fixed directly in the control panel.

Keep me posted. I would be glad to know what the result would be.

Regards,