Nest Thermostat has power or battery problems

The checks in more detail

The fix, in order, is to check the wires at the thermostat base are seated firmly in their terminals, to confirm that a C-wire is connected at both ends, to fit Google's Power Connector if there is no C-wire, and to have an HVAC technician check the transformer and the control board if the warning persists. Turn the HVAC power off at the breaker before touching any wire. While you wait for the fix, the thermostat keeps working on its battery for a time, but heating and cooling may be affected when the battery runs flat.

Keep the display on a low setting and avoid frequent Wi-Fi reconnects, which use power. After wiring is corrected, the battery recharges over a day, and the warning clears. A thermostat that shows a low battery after a reset has the same cause. Check the Google Home app's thermostat details for the battery level and the power status. If you installed the thermostat yourself and the warning started afterwards, re-check the wiring against the app's diagram, since a swapped or loose wire is the commonest mistake.

A professional installation costs less than a replacement thermostat or a damaged furnace board, so call one if the problem is beyond a loose terminal. Note that some systems, such as certain heat pumps and boilers, need specific wiring or an adapter, and a thermostat installed without the right wire may seem to work for weeks and then show the battery warning. Check Google's compatibility checker for your system and the wire guide in the app. If you are unsure, a technician can add a C-wire or fit a transformer in an hour, which is a small cost compared with a damaged board.