Nest Thermostat will not complete setup or pairing

The checks in more detail

After mounting, the thermostat shows a setup screen with a code or a QR. In Google Home, tap Add, Set up device, and scan the code. The phone needs Bluetooth, Location and Nearby devices permissions on Android, or Bluetooth and Local Network on iPhone, and must be on the Wi-Fi network the thermostat will use. Google's setup page lists the connection checks. If the app cannot find the thermostat, check that it shows the setup screen and not the home screen: a thermostat that was set up before needs a reset from its Settings menu.

Use the 2.4 GHz band if the thermostat fails on 5 GHz, and a guest or isolated network blocks setup. A weak signal at the thermostat's wall position makes setup fail partway, so test the signal there with your phone. Keep the phone within a few meters. If setup works but the thermostat then goes offline, move the router or add a node. If the app reports a wiring problem, such as a missing wire, follow the suggestion, which may be to fit the Power Connector, or call an HVAC technician.

Do not force wires into terminals. Complete the system test in the app to confirm that heating and cooling respond. Take a photo of the old thermostat's wiring before you remove it, and label each wire with the tape that comes in the box. Note which terminals hold which wires, because the labels on old thermostats may not match the new one. After mounting, check that the display lights, and if it does not, check the breaker, since many installers turn off the HVAC breaker and forget to switch it back on.