Nest Thermostat Won't Turn On? (Possible Causes & Fixes)

Brigid Levi
by Brigid Levi

Picture it: Winter, 2021. You live in a cold-climate area and temperatures are dropping. You decide to ask your Nest thermostat to kick up the heat a few degrees and . . . it won’t even turn on!

Nest won’t turn on if the battery is too low. This is indicated by a blinking red light. If your Nest is blinking green and won’t turn on, it may have frozen during a software update. When the battery’s completely drained, the screen goes blank with no apparent lights. Nest won’t turn on if there’s a wiring or Wi-Fi connection issue.

Unfortunately, unresponsiveness is a common occurrence with Nest thermostats. Knowing the reason behind why it happens is half the battle. The other half is how to troubleshoot these issues.

Do You Need Your Thermostat Repaired or Reprogrammed?

Get free, zero-commitment quotes from pro contractors near you.

Motion Sensor

When Nest doesn’t sense motion, it goes into standby mode, and the screen appears blank. It activates when it senses someone is nearby or if someone adjusts the thermostat from the app.

If neither of these things activates the screen, consider it unresponsive. You may notice a red or green blinking light accompanying the blank screen. Or there may not be a light at all. The app may also indicate a wiring or Wi-Fi issue. All of these are helpful clues to determine what’s wrong with your Nest.

Blinking Red Light and Not Turning On

A blinking red light on your Nest means the battery life is very low, though it is still receiving a charge. To troubleshoot this issue, you can do several things.

1. Remove Nest from the wall and charge it via the USB port.

Your Nest came with a USB cable and plug. Depending on the model, you’ll see either a mini or micro USB port on the back of your Nest. Google does not suggest using a computer to charge the device.

Depending on the battery’s level, it could take anywhere from a few minutes to a few hours to charge completely. The screen will automatically turn on once it’s charged. You’ll receive a message asking you to reconnect the thermostat back to the base. Press it in firmly and listen for the click.

Please note that while charging Nest this way, your HVAC system remains unusable.

2. Plug Nest into a wall or add a C wire

If the blinking red light keeps reappearing, manually charging the battery may get old. For a more permanent solution, you can plug your Nest into a wall outlet. Or you can add a C wire.

A C wire gives your thermostat a constant stream of power, which it needs since it’s connected to Wi-Fi. Without a C wire, Nest receives power from the HVAC system when the system is running. Over time, the battery may drain from inconsistent charging. The result is a blinking red light.

3. Check for an HVAC issue

Without a C wire, your Nest relies on power from the HVAC system. If there’s a problem with the system, this directly affects how Nest gets its charge.

Air conditioners and furnaces are equipped with safety features that automatically turn the unit off if it senses a problem. This could happen for any number of reasons like a clogged air filter or excess water build-up.

If there’s an issue with the HVAC system, oftentimes Nest sends alerts on the thermostat screen, in the app, or in your monthly Home Report email.

Blinking Green Light and Not Turning On

Nest’s thermostat display shows a blinking green light when it’s updating software, starting up, or restarting. The light should only last for a few minutes. After that, the thermostat should function normally.

If Nest is still blinking green after 15 minutes and the screen won’t turn on, the device may have frozen during an update. Take the following steps to troubleshoot this issue.

1. Disconnect and reconnect the display from the base

It’s possible your Nest wasn’t properly connected to the base plate. Remove Nest from its base, making sure you pull it off straight. Wait a few seconds. Make sure the pins are lined up correctly and press the display firmly back into the base. You should hear a click.

2. Power-cycle your Nest

Power-cycling simply means to turn something off and then back on again. It’s a simple and effective way to reset a device that might only need a quick jolt to its system.

With the display on the base, press the Nest ring down firmly for 10 seconds. Then, release it. You’ll see the Nest logo appear as an indication that the device has restarted.

No Light and Not Turning On

If there are no blinking lights on the Nest display but it won’t turn on, the battery has most likely drained completely. There could be several reasons for this.

1. The display isn’t properly connected to the base

The display and base connection is how the battery receives its charge. If the thermostat isn’t properly seated on the base plate, the battery can’t get enough power.

  • Remove the display from the base, making sure you pull it out straight.
  • Push in any cables that may be sticking out too far.
  • Line up the connector pins and press the thermostat firmly into the base. Listen for the click.
  • At this point, you may see the blinking red light, indicating your battery is low but charging. It may take a few hours to completely charge and will turn on when it reaches that point.

2. Your system isn’t receiving power

Unless you have a C wire, the HVAC system charges your Nest. So, if your HVAC system isn’t powered on, it’s safe to assume your Nest battery will eventually drain.

  • Check your breaker box to ensure the HVAC switch (or switches) is fully in the “On” position. If so, you can try flipping them off and then back on to see if that resets the issue.
  • If you have a fuse box, make sure the fuse controlling the HVAC system isn’t blown. Replace it if needed.

3. Your HVAC system isn’t compatible with Nest

Nest is compatible with most 24-V heating and cooling systems. It is incompatible with the following:

  • Proprietary systems
  • Millivolt heaters
  • High voltage systems
  • Solid fuel systems

If your Nest is connected to one of these incompatible systems, it’s not receiving the proper amount of power. Connecting a Nest to one of these systems can damage the device.

4. The wiring is incorrect or damaged

Before checking the wiring on your Nest, turn off the power to your HVAC system at the circuit breaker panel or fuse box.

  • Remove Nest from the wall to expose the wires.
  • Ensure that all the wires are in their correct terminals with the connector tabs pressed firmly down.
  • Remove one wire at a time from their terminals, checking for corrosion.
  • Reinsert the wires, making sure to press the tabs firmly back in place.
  • Restore power to the HVAC system by turning on the breaker.
  • Replace Nest back into the wall and wait for it to fully power on.

5. There’s a blown fuse in your furnace

To check if your furnace has blown a fuse, shut off the power at the breaker. You’ll notice burning or discoloration if the fuse has blown. In this scenario, it’s best to ask a professional to replace it unless you feel highly confident.

6. Your Nest is damaged

In rare cases, your Nest might not be turning on because it’s damaged. You can test for this by reinstalling your old thermostat. If once you’ve installed it, the old thermostat receives power, the problem is with your Nest. At this point, you may need a professional to help install your Nest properly.

If the old thermostat does not receive power, the problem lies with your HVAC system. Check that the power to your system is on and the fuses in the control board are intact. If all is well from what you can see, ask for a professional’s opinion.

You can also contact Nest support. They can walk you through some troubleshooting options without reinstalling your old thermostat.

Error Message on Thermostat or App

If you receive an error message on either your thermostat or in the Nest app, this indicates a problem with wiring or Wi-Fi connection. You have to look up the code and troubleshooting instructions for your exact Nest model.

Do You Need Your Thermostat Repaired or Reprogrammed?

Get free, zero-commitment quotes from pro contractors near you.

Related Questions

How long does it take for a Nest thermostat to restart?

Some of the above issues require you to restart your Nest. If this is the case for you, you can expect Nest to take roughly 1-2 minutes to restart. If it takes longer, check that the base and display are firmly connected.

What happens when you reset a Nest thermostat?

Depending on the option you choose, resetting a Nest thermostat erases some or all of your personal information and settings. However, when restarting a Nest, the information remains intact.

Summing It Up

While it’s frustrating when your Nest won’t turn on, rest assured that there’s a reason and that reason is fixable. Check for all these scenarios and try your hand at troubleshooting before calling in the professionals.

Brigid Levi
Brigid Levi

Brigid Levi is a wife, mother, and freelance writer who enjoys a good DIY project and creating beautiful spaces within her home. From cleaning and organization hacks to home decor ideas, she loves helping people in their quest to turn a house into a home. Her hobbies include pretending to be Joanna Gaines while updating her home with her husband and performing in local theater productions.

More by Brigid Levi