Nest thermostat flashing red light
On a Nest Thermostat E or a Nest Learning Thermostat (3rd gen or earlier), a blinking red light on the display means the battery is very low but is charging. Google says to wait until it is fully charged, which can take several minutes or up to an hour, before you rely on it to control your system.
What does the red light on a Nest thermostat mean?
It is a charging light. On the Nest Thermostat E and on the Nest Learning Thermostat (3rd gen or earlier), Google says a blinking red light at the top of the display means the built-in battery is very low and is being charged right now.
These models run on a rechargeable battery that the thermostat charges with power drawn from your heating and cooling system. The red light tells you two things: the charge has dropped very low, and charging is already under way. Google's instruction is to let it finish: do not use the battery to run your heating or cooling until it has fully charged.
Which Nest models show a red light?
Only some. Google uses different signals on different generations, and one model has no light at all:
| Model | Very low battery, charging | Blinking green | Charging port on the back of the display |
|---|---|---|---|
| Nest Thermostat E | Blinking red | Updating software, starting up or restarting | Mini or micro USB, depending on the model |
| Nest Learning Thermostat (3rd gen or earlier) | Blinking red | Updating software, starting up or restarting | Mini or micro USB, depending on the model |
| Nest Learning Thermostat (4th gen) | Dark yellow light; Google says it does not blink while charging | Not stated | USB-C |
| Nest Thermostat (2020) | No light. Runs on 2 AAA alkaline batteries | No light | Not stated |
How long does a Nest thermostat take to charge?
Google gives two answers on two pages. The troubleshooting page for a thermostat that will not turn on says charging can take several minutes or up to an hour, depending on how low the battery is. It adds that if you have waited more than an hour and the thermostat still has not come on, something else may be wrong.
Google's page about the USB port says recharging usually takes about half an hour, and up to 2 hours when the battery is fully drained. Google does not explain the difference. If you are charging from the system, give it the full hour before you start to worry.
How can you charge it faster?
Google tells you to pull the display off its base and charge it through the USB port on the back, using a USB cable and a wall charger. The Thermostat E and older Learning models take mini or micro USB depending on the model; the 4th gen Learning Thermostat takes USB-C.
Use a wall charger. Google's troubleshooting page warns against a computer USB port or any other unpowered port. Its USB port page says a computer will work, but the safer instruction is the one to follow.
Once the screen is back, you can see whether the battery is holding a healthy charge. Open Quick View, then Settings, then Technical Info, then Power. Google's information menu page says the battery reading should be higher than 3.6V while it charges normally. Below that, the thermostat turns off Wi-Fi, holds back software updates and disables the screen. A software update needs 3.7V.
Why does the red light keep coming back?
If the light shows up again after weeks of normal use, Google says the cause may be the thermostat wiring or another power issue. In that case the red light can appear and disappear by itself over and over. Wiring and power supply problems are not something to fix from the thermostat; a licensed HVAC technician should look at them.
Google also points out a less obvious cause. A clogged air filter can make the heating or cooling system overheat and shut itself off, and while the system is off the thermostat cannot draw power to charge its battery.
What if the light is green, or there is no light?
A blinking green light on the Thermostat E or a 3rd gen or older Learning Thermostat means it is updating its software, starting up or restarting. Google says this normally lasts a minute or 2. If it blinks green and stays unresponsive for more than 15 minutes, it may have frozen during the update; Google's step is to remove the display, reconnect it and restart the thermostat.
No light at all, with a screen that will not wake, means the battery has drained completely. Charge it as described above. For a dark screen on any brand, see thermostat screen is blank, and for the low battery warnings Nest shows before it gets this far, see Nest thermostat low battery.
What should you do now?
Has the light been blinking red for less than an hour?
- Yes
Leave it to charge. If you want it back sooner, take the display off and charge it by USB from a wall charger.
- No
After charging by USB from a wall charger, does the screen come back?
- Yes
Check Settings > Technical Info > Power. If the red light keeps returning after normal use, Google points to wiring or another power issue, which is a job for a technician.
- No
Google says something other than the battery may be wrong. Check the signs at the bottom of this page.
- Yes
Once the thermostat is running again, a dark screen is not always a battery problem. Google says the display turns black when the thermostat is off or in Eco, so check whether it is in Off mode or using its Eco temperatures. For other Nest messages and settings, see the Google Nest hub.
Frequently asked questions
- Can I use my Nest thermostat while the red light is blinking?
- Google advises against it. Its troubleshooting page says to wait until the battery has fully charged before you use it to control your heating or cooling. Leave the thermostat alone until the light stops and the screen comes back.
- Can I charge my Nest thermostat from a laptop USB port?
- Follow Google's troubleshooting page, which says not to use a computer USB port or any other unpowered USB port. Use a USB cable and a wall charger. A separate Google page on the USB port says a computer works, but the troubleshooting page is the more cautious instruction.
- Why does my Nest Learning Thermostat (4th gen) show a yellow light instead of red?
- On the 4th gen, Google uses a dark yellow light for the same situation: the battery is very low and is being charged. Google's USB port page also notes that the 4th gen does not blink while it charges.
- Does the Nest Thermostat (2020) have a red charging light?
- No. It has no charging light and runs on 2 AAA alkaline batteries. When they get low, it turns off features such as Wi-Fi and motion sensing, and you may need to tap the right side to wake it.