Article Summary:
The Nest Learning Thermostat will turn on automatically after you’ve properly connected your heating and cooling system wires and turned the power back on. If your thermostat doesn’t turn on, try the steps below to troubleshoot.

There are a few possible reasons why the Nest Learning Thermostat isn’t turning on as expected. Below are detailed descriptions of each possible reason and suggestions for how to get them fixed. But you should first try our online Troubleshooting tool. It’ll walk you through isolating the problem and getting it fixed right away.

1) Did you turn the power back on to your heating and cooling system?

Sometimes breaker box switches can be tricky. You may think you flipped it back on, but may not have flipped all the way.

Try turning the breaker or power switch to your heating and cooling system back off and on again. If your Nest Thermostat starts up (it can take a few seconds), you’ll know that was the problem.

2) Is there a flashing red light on the display?

A small, flashing red light on the Nest Thermostat display means that the battery is low and is charging. Once charging is complete, your thermostat will turn on.

Why is there a flashing red light on the Nest display? >

3) Is there a flashing green light on the display?

A small, flashing green light on the Nest Thermostat display usually means that it’s updating software. It should automatically turn on when finished. But if this continues for longer than a few minutes, see the article below for more troubleshooting.

Why is there a flashing green light on the Nest display? >

4) Is the Nest Thermostat display fully connected to the base?

The Nest Thermostat won’t turn on until the display is properly connected to the base. If the wires connected aren’t flush with the base, they may be preventing the display from getting a solid connection.

Try removing the display and pushing the wires back into the wall as far as you can. There shouldn’t be any wires sticking out beyond the base. Then reconnect the display. If that was the issue, the Nest Thermostat will power on.

5) Are the wires connected correctly?

Important note: Be sure to turn off the power to your heating and cooling system before you try any troubleshooting steps that involve removing wires. This will prevent your system from blowing a fuse.

Incorrect wiring may prevent the Nest Thermostat from turning on. Before you check your wires, turn the power to your heating and cooling system off. Next, remove the Nest Thermostat display and take a look at the wiring to check the following:

You can just continue without the common wire installed; the Nest Thermostat usually doesn’t need it.

The exposed part of the wire should be ⅜” (1 cm) and should be straight. You should clip and re-strip any crooked or mangled wires.

When you reconnect the wires, simply push down on the connector, insert the wire all the way, and then release. If the connector button doesn’t stay down, the wire is not fully inserted.

The Nest Thermostat only works with 24 Volt systems, so millivolt systems aren’t compatible with the Nest Thermostat. Once you confirm that your system is 24V, move on to the next step.

How can I tell if my current thermostat is low voltage? >

Do you have a common wire in the C connector?

If you have a common wire connected to the C connector, start by removing it and turning the power back on. If your Nest Thermostat starts up, that means that the common wire that was installed in your old thermostat was not delivering voltage, either because it wasn’t hooked up or because of corrosion in the wire.

Are the wires inserted correctly?

The Nest Thermostat won’t start up if the wires aren’t correctly inserted into the connectors. To check, make sure the power to your heating and cooling system is off and then remove, inspect, and replace each wire one at a time (so that you remember where they all go).

Do you have a millivolt system?

If you have only two wires connected to the Nest Thermostat base, there is a chance that your system is a millivolt system. This type of system can’t provide any power to the Nest Learning Thermostat.

6) What else can I do?

In some rare cases, the battery may have drained during shipping. If you see a blinking red light when you attach the Nest display to the base, that means it’s charging. It usually takes a half an hour, but can take up to a couple of hours if the battery is completely drained. You can charge your thermostat quicker with a USB cable.

What you’ll need:

• Micro-USB cable for 3rd and 2nd generation Nest Learning Thermostats
• Mini-USB cable for 1st generation Nest Learning Thermostat
• A computer with a powered USB port or USB power adapter

Here’s how:

1. Remove the Nest Thermostat display from the base.
2. Plug the small end of the USB cable into the port on the back of the Nest display and the other into a power source.

Once the screen lights up and you see a message asking you to attach the Nest Learning Thermostat to the base, unplug the USB cable from the back of the display and reattach the Nest Thermostat display to the base. From there, follow the onscreen instructions.

If there is a problem with your wiring, your Nest Thermostat will provide details and direct you to troubleshooting tools to get the problem resolved.

7) Does your old thermostat still work?

The next step is to reattach your old thermostat. Turn the power back on and verify that your heating and cooling system is working properly. If your heating and cooling system works with your old thermostat, use our Troubleshooting tool to help isolate and fix the problem.

If your heating and cooling system doesn’t work with your old thermostat either, there is probably an issue with the electrical supply to your heating and cooling system. It could be a blown fuse on the furnace or a bad circuit breaker, and we’d recommend contacting a Nest Pro Installer.