3-Wires Thermostat to Nest Gen 3 wiring - e74 error

[OlgaVolga]

I bought a Gen 3 learning thermostat and can't correctly rewire it with the existing cables. There are only 3 cables: red, black and a copper one(no insulation on it). I'll attach the photo of the original thermostat where they where labeled with colors: Yellow(signal), Red (12V) and Black(Gnd) in the comments. 

I wired Yellow to Y1, Red to RH and Black one to C - I keep on getting an error e74 that no power is going to Rh wire. 

Please help,

[Markjosephp]

Hello OlgaVolga,

Thanks for reaching out here in the Google Nest Community Forum. I'd be happy to take a look into this for you.

Based on your description, it seems like you've connected the wires to the Nest Thermostat correctly, matching the colors with their respective labels. However, the message suggests a power issue with the RH wire. Your C-wire might not be properly grounded, so it can't complete the circuit.

Here are a few things you can check:

1. Ensure that your HVAC system is supplying power to the RH wire. Some systems may require a common wire (C) for continuous power. In your case, you've connected the Black wire to C. Double-check that the power is flowing through the system. Let us know its power readings in the Technical Info option.
2. Check if there's a fuse or circuit breaker associated with your HVAC system. If it's blown or tripped, it could be the reason for the lack of power.
3. While you've mentioned connecting Yellow to Y1, Red to RH, and Black to C, it's worth verifying directly on your HVAC system control panel. Make sure there are no loose connections and that each wire is securely attached.
4. Use a multimeter to measure the voltage between the RH and C wires. This will help you confirm if there's indeed power coming through the RH wire.

If the issue persists after checking these steps, please share with us a photo of your old thermostat's wiring setup.

Let me know how it goes.

Best regards,
Mark