Troubleshoot when your thermostat won’t turn on

If your Google Nest thermostat won’t turn on and you see a blinking red or green light, or no light at all, there could be a power issue or your thermostat may have frozen during a software update. Here’s how to troubleshoot.

If there’s a power issue, your thermostat’s battery will drain and it will turn off Wi-Fi, the display and other features to preserve battery life.

Before you start troubleshooting:

  • Your Nest thermostat should automatically turn on once it’s connected to your system and your system is powered up.
  • If you see a blinking red light at the top of your Nest thermostat’s display, you’ll need to wait until the battery is charged before you can use it.
  • During regular use your thermostat’s screen may automatically turn off after a period of inactivity, but it should also automatically turn back on when it senses someone is nearby or when someone uses the app to control it.
  • Your Nest thermostat uses the voltage from your system’s wires to keep its internal battery charged. If the power to your system is switched off or there’s a blackout, the battery has enough capacity to keep working for a while. Your thermostat will also turn off features like Wi-Fi to preserve battery life before it must turn off completely.
  • If you see a Wiring Report or error message on your thermostat or in the app, follow the troubleshooting steps in Troubleshooting Nest thermostat error codes.
  • Use the troubleshooting steps below only if your thermostat is unresponsive when you turn the ring or press the screen.

1. If there's a blinking red light on your thermostat 

A blinking red light near the top of your thermostat’s display means that the battery charge is very low but it’s being charged. There are different troubleshooting steps depending on when you see the red light.

Troubleshoot during installation

  1. Your thermostat should automatically turn on.

    Your thermostat should automatically turn on when the battery is charged enough. Depending on how low the battery is, you may only have to wait several minutes. But an extremely low battery can take up to an hour to charge.

2. Speed up charging with a USB cable

  1. Pull off the Nest display and use the USB port on the back to charge it with a USB cable and wall charger. Depending on the model you have it will be either mini or micro USB. Note: Do not use a computer USB port or other unpowered USB port for charging.
  2. The the display should eventually light up with a message asking you to connect it to the base.
  3. Reconnect the display to the thermostat base. Make sure you press firmly until you hear it click into place.

3.  If the blinking red light doesn’t go away or keeps coming back

A persistent blinking red light can mean a couple of things. You’ll need to check the power readings on your thermostat.

  1. Charge your thermostat with a USB cable if needed.
  2. Reconnect the display to the thermostat base and go to Settings > Technical Info > Power.
  3. If the Voc, Vin or lin values are not within the specific ranges listed here, there may be a wiring or system issue:
    • You may need to connect a C wire to your thermostat. If there’s a C wire connected to your system’s control board, you can put it in the thermostat’s C connector to see if that helps. Here are instructions on how to check.
    • There’s a problem with your system and it may need maintenance for a variety of reasons. For example, a clogged air filter can cause your system to overheat and automatically shut off. When this happens, your Nest thermostat can't draw power from your system to charge its battery. You may also see a Furnace Heads-Up message on the thermostat screen, in your Nest app messages, or in your monthly Home Report email. Contact a local pro to service your system and check your thermostat wiring.

Troubleshoot after installation

If you’ve successfully installed your Nest thermostat, but a blinking red light appears after you’ve been using it regularly for some time, there may be a problem with the thermostat wiring or another power issue. For instance, the blinking red light may show up then go away on its own repeatedly.

Skip to section 3 below and follow the instructions for checking that the wires are in the right thermostat connectors and are properly seated, and to check that there aren’t issues with your system.

2. If there's a blinking green light on your thermostat 

A blinking green light near the top of your Nest thermostat’s display means that it’s updating the software, starting up or restarting. Typically, this will only last a minute or two. Once your thermostat has finished starting up, it will turn on so you can use it.

  1. Try removing and reconnecting the display. Pull it straight off the base. Make sure the pin connector is lined up correctly and push it back onto the base. You should hear a click to confirm it is properly seated.
  2. If reconnecting the display doesn't help, press down on the thermostat display for 10 seconds then release it. This will force your thermostat to restart. For more details, please see the following article:

How to restart or reset Nest thermostats

3. If there's no light on your thermostat 

If there’s no blinking light on your thermostat, and it’s unresponsive and won’t turn on, the battery has drained and there’s not enough power being delivered over your system’s wires.

1. Check that the display is properly connected to the base

If the display isn’t fully seated on the base, your thermostat won’t turn on and can’t charge its battery.

  1. Pull the thermostat display straight off the base.
  2. Make sure that there aren’t any cables sticking out from the wall that might prevent the display from being fully seated. If there are, press them back into the wall.
  3. Line up the display and base connectors and press the display straight onto the base until you hear it click into place.
  4. If your thermostat shows you a blinking red light, the battery is charging and it will eventually turn on. It can take up to an hour if the battery is severely depleted.

2. Check that your system is receiving power

  1. Try turning the breaker or switch that controls power to your system back off and on again. Make sure the switch is pushed all the way to the On position.
  2. If you have a fuse box, check that the fuse controlling power to your system isn’t blown and is working normally. Replace it if needed.
  3. If your thermostat shows you a blinking red light, the battery is charging and it will eventually turn on. It can take up to an hour if the battery is severely depleted.

3. Double check your system compatibility

Connecting a Nest thermostat to an incompatible system, such as a millivolt, proprietary or high voltage system, won’t deliver the power needed to charge your thermostat’s battery and can damage your Nest thermostat.

4. Check your thermostat’s wiring

Symptoms caused by wiring issues usually occur when you're first installing your thermostat. 

Important: Turn power to your system off at the system switch or fuse box. This helps protect your system and thermostat from possible damage.

Check that each wire is fully inserted into the base and is corrosion free.

  1. Pull the thermostat display off the base. 
  2. Pull out one wire and inspect it.
    • The exposed part of the wire should be clean, straight, and 3/8” (1 cm) long. Cut and re-strip any crooked or corroded wire ends.
  3. Re-insert the wire. The connector button should stay down. If it’s not, the wire isn’t fully inserted. Repeat the above steps with each wire.
  4. Once you've checked all the wires, push the display onto the base until you hear a click.
  5. Turn the system power back on.
    • If your thermostat shows you a blinking red light, the battery is charging and it will eventually turn on. It can take up to an hour if the battery is severely depleted.
    • If you don’t see a blinking red light, it's possible that you’ve put wires into the wrong thermostat connectors. Make sure you have entered your system’s wires correctly into the Nest app and follow the wiring diagram it gives you.

5. Your system may have a blown fuse

A blown fuse on your system’s control board can prevent power from reaching your thermostat.  Contact a local pro to check your system and your thermostat wiring.

6. Check if your Nest thermostat is damaged

In rare cases, a Nest thermostat can be damaged by things such as a power surge from your system, connecting it to an incompatible system, or by touching exposed wires together if power to the system hasn’t been turned off during installation.

You can test for this one of two ways.

Try reinstalling your old thermostat

If your system works as it should with your old thermostat, your Nest thermostat may be damaged and needs to be replaced.

  1. Turn off power to your system at the breaker box or system switch. This will help protect your system and your thermostat from possible damage.
  2. Pull your Nest thermostat off the base and take a picture of the wires for reference. 
    Note: If you still have it, use the picture of the wires in your old thermostat that you were instructed to take during thermostat installation .
  3. Uninstall your Nest thermostat and reinstall your old thermostat. Make sure that the wires are placed in the right connectors.
  4. Turn the power to your system back on and test that it’s working properly.
    • If your system works with your old thermostat, try getting a professional to install your Nest thermostat. To get a professional to help install and set up your Nest products, contact our installation partner, OnTech. If your Nest thermostat is damaged, or if you need a professional to service your system, the installer may let you know.
    • If your heating and cooling system doesn’t work with your old thermostat either, there is probably an issue with your heating and cooling system. It could be a blown fuse on the furnace or a bad circuit breaker, and we’d recommend for you to contact a local pro.

Contact our support team

They can walk you through steps to test your Nest thermostat without reinstalling your old one. Contact Nest support.

Contact a pro for help

If these steps don’t resolve the problem, you should contact a pro for other issues (including HVAC). The problem may be caused by an issue with your system.

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