e74 error Nest Learning Thermostat Gen3

[gcampbell]

My Gen3 Nest Learning Thermostat always give an e74 error, saying there's no power to the Rh wire. The previous Honeywell thermostat (wiring photo attached) has wires to Y, G, C, O/B, and R and works just fine.

I turned off power and connected to the same terminals on the Nest (with the red R wire connected to Rh), then turned power back on. I even waited a fairly long time to be sure the 5-minute wait period since the condenser last powered off had elapsed. The Nest diagram claims there is no power to Rh. The voltage Nest registers is essentially zero (photo attached). 

When I turn the power back off, reconnect the Honeywell thermostat, and turn power back on, everything works again immediately. The fuse on the AC control board has not blown. There clearly isn't a float switch or other power problem since it immediately works fine when I connect a less-sophisticated thermostat to the same wires.

The AC unit is a GrandAire WPH4 14 SEER package heat pump.

Any ideas what might be the problem? Since we're getting to 95F pretty regularly soon, I'll probably just stick with the basic Honeywell thermostat for the next 4-5 months unless someone has a good idea.

[zoeuvre]

Hi gcampbell, 

Thanks for reaching out to us, and I'm sorry to hear about the situation. Let's sort this out. 

Turn off the power to your HVAC system again, then remove and reconnect your thermostat wires one by one to the Nest Learning Thermostat base.  If the same thing occurs, kindly charge your Nest Learning Thermostat display using a micro-USB cable for an hour then observe the Nest Learning Thermostat after that since it also shows a low battery of 3.686 V. 

Let me know how it goes. 

Best,

Zoe

[zoeuvre]

Hi there,

Checking back in — did you get a chance to try the steps I shared above? If so, how did it go? 

Thanks,

Zoe

[gcampbell]

It's too hot out to be playing around with the thermostat. If there's a cool day, I try it out, otherwise I'll probably try again in October.

[zoeuvre]

Hello gcampbell, 

Thanks for the follow-up. You can still get back in this thread for your reference of the troubleshooting steps I shared so that once you're available to do it, you'll have something to follow. On the other hand, I'll go ahead and lock this thread for 24 hours. If you happen to have the same concern, feel free to reach out to us here or start a new thread.

All the best,

Zoe