My nest thermostat worked fine until one month ago, now it heats in cool mode

[bsec]

I have two Nest Learning Thermostats installed several years ago and they had been working fine until one month ago. One thermostat now tried to heat while setting in cool mode, I did factory reset, power off/on, it would work for a day or two then back to the same problem. 

Did anyone run into the same problem? Any fix?

[bsec]

Forgot to mention, the other thermostat still works.

[AxelD]

Hi bsec,

Sorry to hear about this situation with your Nest Thermostat. No worries, help is here! Let me ask you a few questions:

• Do you have a C-WIRE or Nest Power Connector installed?
• What kind of system do you own? E.g. Conventional, Heat Pump.

In the meantime, I'm leaving you  with this article Troubleshoot strange heating or cooling behavior that contains some troubleshooting steps.

Keep me posted.

Regards,

Axel.