Nest Thermostat Won't Turn On and Blinking Lights

In this article, I go through what to do when your Nest Thermostat won’t turn on and what it means when the blinking lights are green or red.

I cover Nest troubleshooting steps to diagnose and repair some of the more common issues that people experience with their Google Nest Thermostats.

These troubleshooting steps cover the Google Nest Learning Thermostat, Nest Thermostat E, and the Nest Thermostat.

Table of Contents show

Nest Thermostat Won’t Turn On

Like all new technologies and devices, Google Nest thermostats are great when they work and a pain when they don’t.

Luckily, Nest products rarely have issues, but from time to time they can suffer from glitches. Frequently these can be resolved in a few simple steps that most people will be comfortable doing.

One note before getting into the details. If you don’t have a blinking color light on your thermostat display and your Nest won’t turn on, it means there’s no power to thermostat.

If there’s no blinking light, you’ll want to check the Nest for battery issues or potential thermostat faulty wiring issues. The wires might be in the wrong thermostat connectors or there’s a loose wire that’s not fully in one of the thermostat connectors.

Nest Thermostat Blinking Green

Like most smart devices on the market, Nest has frequent software updates that usually only take a few minutes to complete. There are also occasional times you need to update firmware.

The green blinking light indicates that the thermostat is in the middle of performing an update. It’ll restart on its own after completing the update.

Occasionally, they have been known to freeze during this process. If the green light has been flashing 15 minutes or more, it’s time to take action.

First thing to check is the Internet connection, because these are Wi-Fi devices after all. Maybe there was a break in the connection and it stopped the download. If the wireless connection looks good, then move on to a couple other possible fixes.

Solution 1

Remove the Nest from the base wall housing and reset the Nest thermostat.
Grasp the edges of the unit and gently pull it away from the wall.
Before you reset it, clean the terminals if they’re not already. This can be done by cleaning with a non-nylon paintbrush, a blast of compressed air, or a substance like WD40 can also be used.
Also check the terminals on both the power socket and on the plug on the back of the thermostat then reset it.
It should click when it’s properly fitted.

Solution 2

If this hasn’t worked then the next step is to force a restart.
To do this, press and hold the display for around 10 seconds.
Once the screen has gone blank, release the button. A message saying ‘The Thermostat is Restarting’ should appear.
One word of caution about this method, this is the equivalent of turning a computer off by pressing the power button only. There may be a loss of some data like settings.

Nest Thermostat is Blinking Red

This message indicates that the Nest battery is low and the battery is currently being charged.

The red light can be caused by different issues depending on when the red light appears.

If it’s a newly installed unit, then the light will generally stop on its own after a few minutes. However, if the red light persists and the unit won’t power on, then there are a few steps you can take to try to rectify the problem.

Because the problem is likely to differ due to the installation status of your thermostat, I’ll cover those situations too: a new installation than it is for an existing one and previously functioning install I will cover this in different sections.

Troubleshooting Blinking Red Light During Installation

If you are experiencing this issue as you’re running through the installation process, the first thing to do is to simply give it some time to charge the battery.

In most instances this will only take a few minutes, but it is worth waiting for at least an hour before taking further action. If it still doesn’t work after this then try the following.

Manually Charge the Thermostat

To do this remove, the Thermostat from its wall housing by pulling outwards away from the wall.
On the base of the unit, there is a USB port. Depending on the model you have, it’s a Micro or Mini USB port.
Using a USB power cable, charge the unit until the display shows a message telling you to connect the thermostat to its base.
Do so by firmly pressing the thermostat into the base until you hear it click into place.

This should have resolved the issue. But if the problem hasn’t been fixed or re-occurs after a short while, then it’s likely the battery isn’t receiving any power from your HVAC system.

Check Nest Battery Status

Check the internal battery life and current battery charge by entering the quick menu and navigating to – Settings -> Technical Info -> Power.

Here you can check that the thermostat is receiving the correct power to charge the battery properly.

There are two sets of criteria to check and which one is relevant depends on whether there’s a C-Wire.

Systems Without a C-Wire

Voc: – 29v to 42v
Vin: – 6v to 7v when your system is running or 29v to 42v when it isn’t running
lin: – 20mA to 40mA

Systems with a C-Wire

Voc: – 29v to 42v
Vin: – 29v to 42v
lin: – 100mA or 200mA

If the figures you see are not as listed, then there is a problem with the thermostat receiving enough power to charge your battery.

If there’s no or little power, check that there’s no obstruction between the battery and the battery terminals. Another thing to check is to make sure the battery’s positive “+” and negative “-” ends are matched correctly to the markings on the battery compartment.

If those look good, it could be the current battery drainage is depleted or it’s a faulty battery. You can try to replace the battery to see if that will help.

Before you proceed further, check the compatibility of your heating system using Nest’s useful checker. It could also mean a compatibility issue.

Troubleshooting Blinking Red Light after Installation

The first thing to try in these instances is the manual charging procedure I described above. It’s also worth cleaning the terminals on the thermostat and its housing, because dust could be stopping the battery from charging.

If the problem reoccurs after this, then try the following steps. These steps can also be followed if there are no lights blinking and the thermostat won’t turn on. In these instances it may mean your battery is completely drained.

Check the basics, make sure the heating system fuse is okay and the HVAC system hasn’t been inadvertently turned off
Check the wires are still attached to both the thermostat and the HVAC boiler. It’s important that you ensure the system is completely turned off before attempting this stage.
Check the readings on the power section of the thermostat and compare the results to the criteria listed above.
If you are confident with a multimeter, you can use this to check that power is getting to your thermostat (multimeter example).

Troubleshoot Nest Thermostat Error Codes

There are times where you’ll see a wiring report or error codes on the thermostat or on the Nest app. These usually indicate that there are power issues with the wires.

The codes will start with one of these letters: T, E, N, TD, M, W, or H. The letters will be followed by numbers.

You can find out more about what each of the error codes on Google Nest’s help page for troubleshooting steps.

One last thing you can do is to see if the thermostat is damaged. If there was a power surge, one wrong connection to an incompatible system, or exposed wires touching together, something could’ve damaged the thermostat. You’ll want to take it in for repairs if this is the case.

If the problems persist after this and you’re confident the power to the unit is okay, then it may mean your unit is faulty or the battery has issues. I have a handy Nest Thermostat low battery troubleshooting article to check out.