e74 error on Nest Learning Thermostat Gen3

[gcampbell]

I'm trying to install a Gen3 Nest Learning thermostat to control my GrandAire heat pump/AC. The previous thermostat was a Honeywell Home ProSeries thermostat and it controls the AC perfectly. When I change to the Nest, it says there's no power to the Rh wire, error E74. I used a multimeter to check voltage, and there's about 25V between the red Rh wire and blue C wire. If I change back to the Honeywell thermostat, it works fine again.  The wires seem correct: Honeywell had R=red, C=blue, G=green, Y=yellow, O/B=orange, and that matches the Nest wiring (connected red wire to Rh, yellow wire to Y1, others labeled exactly the same). Any ideas why it's not working?

[Markjosephp]

Hi gcampbell,

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

Your system may not provide enough power or any power to either the thermostat or a specific wire (so the thermostat can’t complete a circuit and shows a report). Could you please share with us a photo of your thermostat's wiring? This will help us see how they are being labeled. Also, gather the power readings in your thermostat's Technical Info option.

I'll look forward to your response.

Regards,
Mark

[Dan_A]

Hey there,

I wanted to follow up and see if you still need help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Best,

Dan

[Dan_A]

Hey there gcampbell,

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you need assistance or want to discuss topics, feel free to start a new thread.

Cheers,

Dan