A blinking red light will appear if the nests battery is too low to start. If the blinking red light doesnt go away or keeps coming back a persistent blinking red light can mean a couple of things.
Fix Nest Thermostat Not Charging Nest Troubleshooting Guide
Nest thermostat flashing red light. But if the battery is fully drained it can take as long as two hours to recharge. The nest should charge itself within a few hours but you can speed up this process by using a micro usb cable. It should turn back on automatically after the battery is charged. If your thermostat shows you a blinking red light the battery is charging and it will eventually turn on. Usually your thermostat will take about half an hour to recharge. It can take up to an hour if the battery is severely depleted.
The nest displays a message that asks you to remove the nest from the base and then reattach it the nests screen is dark or unresponsive you may see a blinking green or red light at the top of the nest the thermostat animations are very slow and unresponsive. Your thermostat has a red blinking light a red blinking light means that the nests battery is running low. Battery is too low to start. If you dont see a blinking. You can also speed up the process by disconnecting the thermostat display and plugging it into the usb port that came with your device. Youll need to check the power readings on your thermostat.
Youll see a blinking red light at the top of the thermostat display to confirm that its charging. This could be due to a wiring problem or extended power outage.