Hi @ChrisWeir!
Thanks for posting in the community.
I’m sorry for the inconvenience that you've been experiencing with your Nest Learning Thermostat 3rd Gen. I understand that the device was working well but randomly disappeared from your Nest app. When trying to add it again, it says that it's already connected to another account. Let me help you with this.
To better assist you I, need some extra information. I understand that you have already tried some troubleshooting steps. May I know what steps you have done so far? Have you checked if you are connected to the same account that you were using before the device disappeared?
If you are connected to the same account and the device is still not showing up, you can follow the next steps.
- Go to Settings, then Reset, and then Account Connection.
- When the thermostat finishes the reset, open your Nest App and then select Add Product. When the Nest app asks you to scan the QR code, you can remove the device from its base and scan the QR code on the back of it.
- Follow the instructions in the Nest App to connect the device back to your account.
For further information, you can take a look at the following articles: Add your Nest thermostat to the Nest app and Restart or reset a Nest thermostat.
Let me know how it goes.
Best regards.
Daniel.
