Intermittent E74 Error Message

[DanBor]

Installed the Nest Learning Thermostat 3rd Generation in May of 2022. For the most part it works fine, both when cooling and also when heating. However, every once in a while I get an E74 error message. The message usually goes away all by itself (without me doing anything) within 1 minute. Then everything is fine (meaning the heating or cooling will still work) until it happens again. And it only happens when the system is between cycles. It will heat or cool for something like 15 minutes, then it is idle for maybe 30 minutes (this is when I get the intermittent E74 error message), and then the heat or cool will come on again. This normally happens several times a week, sometimes more than once a day. I have checked the wire, checked the float, checked the fuse, and everything checks out fine. Any ideas? 

[Patrick_Caezza]

Try moving it to the Rc terminal and making sure the bare wire is nice, shiny, and straight.

[DanBor]

Patrick, I read that the Nest Learning Thermostats use an internal jumper between the Rh and Rc connectors. And the Nest Learning Thermostat will automatically use this internal jumper if the system needs it (how does it know if the system needs it?). Might that explain why my E74 outage lasts only a short time?

[Markjosephp]

Hello there,

I wanted to stop by and see how I could help.

Thanks for suggesting a step, @Patrick_Caezza.

@DanBor, 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).

To isolate: does your thermostat have a C-wire or a common wire connected? If so, try removing it, leaving it disconnected, and restarting your thermostat to see the changes. Also, please gather the power readouts under the Technical Info section with and without the C-wire.

Please let me know how it goes.

Regards,

Mark

[Jake]

Hey there,

I wanted to check in and see if you managed to see Mark's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.

Best regards,
Jake

[Jake]

Hey there,

I wanted to check in, and let you know that I will be locking the thread in 24 hours.

Best regards,

Jake