Google suggests two potential solutions for the "w5" Nest thermostat problem. The last option, if none of those work, is to ask for a free replacement.

A quick reset might work if your Nest thermostat won't connect to your Wi-Fi network and is powered by software version 6.0 or later.

Go to Settings > Reset > Network on your Nest thermostat to first reset the network settings. Select Settings > Reset > Restart to restart the Nest thermostat. Try to re-connect to your Wi-Fi network once the thermostat is operational once more by going to Settings > Network. Update the system software on your Nest thermostat. Updating the firmware on your Nest thermostat to the most recent version is another potential solution to the "w5" problem.

Choose Settings > Software > Update if your Nest thermostat's display shows the "w5" error indicator and an exclamation point. A new update should automatically install if it becomes available. When it does, navigate to Settings > Network and try to connect to your Wi-Fi network once more. Did you instead encounter the error message "Unable to check for a software update"? If all goes OK, try manually connecting to your Wi-Fi network after choosing Connect.
 Get a replacement for nothing Still not functioning? If this is the case, it might be time to give up and request a replacement. Visit the Nest support page on Google, choose your nation, and start an online chat. The hardware on your Nest thermostat might need to be replaced.

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