Important: The instructions in this article apply to heating and cooling systems in the United States and Canada. If you live in Europe and need to fix your thermostat, check Manually control heating when Heat Link is disconnected first.
Heating and cooling systems can behave strangely. For example, it stays on all the time, or it won't turn on. It might also turn off and on repeatedly, or make strange sounds.
If this happens, your system might have a power issue or a number of other issues may be affecting your system. It may need a C wire (also called a common wire) or the Nest Power Connector to help give steady power to your thermostat. Learn more about Nest Power Connector.
- Your system is acting strangely: If you find one of these problems without an error code, go to step 2 to fix it.
- Your system is making strange sounds, like chattering, stuttering, clicking or thumping. This can be caused by your system turning on and off rapidly
- Heating or cooling is always on, and won’t turn off.
- Heating or cooling repeatedly turns on and off in a short period.
- Your thermostat shows a "Delayed" or "Starts in" message. For example, “Heating is delayed for 2:30 mins”.
- The fan is always on or won’t turn on.
- The fan turns on and off repeatedly in a short period.
- There is an error code on your thermostat or in the app: If your thermostat shows an error code that starts with a letter and has a number, like N80, W4, or E73, go to Troubleshooting Nest thermostat error codes.
- Your system cools when it should heat, or heats when you want cooling:
Go to Troubleshoot when your heat pump heats when it’s supposed to cool. - Your thermostat’s temperature changes unexpectedly: This might mean there's a problem with your temperature schedule. To learn how it works and how to make changes if you need to, go to Nest thermostat temperature schedules.
- Your thermostat goes into and out of Eco Temperature unexpectedly:
- It goes to Eco even if you’re home.
- It stays in Eco even if you’re home.
- It takes a long time to switch into or out of Eco.
If you find one or more of these signs, go to Troubleshooting automatic home and device switching.
- Other signs: If you have a heating or cooling issue not listed above, search our support site for your specific issue. Go to Google support.
Google Home app
- Open the Google Home app
.
- Tap Home
Climate
- Tap Energy dashboard
.
- Choose Day, Week, or Month.
.Nest app
- Open the Nest app.
- Tap your Nest thermostat on the home screen.
- Tap History
to check what happened over the 10-day period.
- Tap a day to learn about why the temperature changed.
On your thermostat
Nest Learning Thermostat:
Open the Quick View menu 
Nest Thermostat E:
Go to Settings 
Learn more about Thermostat Energy History.
Check the symptom lists in step 1. If your symptoms match, you can try these steps to fix them.
Wiring issues usually happen when you first install a Nest thermostat. But they can also show up later, often for other reasons.
- To prevent damage and keep you safe, turn off power to your system at the fuse box or system switch.
- Pull off the Nest thermostat display.
- Make sure all connectors with wires are down (refer to picture).
- Re-insert wires to make sure the connector button stays fully pressed down. Your Nest thermostat won’t know there’s a wire if the button is not down.
- To make sure they go all the way into the Nest base’s connectors, you might need to strip and straighten the wire ends again.
Some common symptoms indicate that your system might need professional maintenance:
- Your Nest thermostat screen turns blue when you want cooling or orange when you want heat, but your system doesn’t turn on.
- Important: When your thermostat’s screen changes color, it’s trying to turn your system on.
- Your system works normally for a while after you restart it but one or more odd behaviors eventually return.
- Try by turning off the power to your system at the breaker switch.
- Wait a few minutes and turn the power back on.
- If the problem seems to fix itself but then comes back after a while, your system might need maintenance.
To check for possible system maintenance issues yourself, you can install your old thermostat and test your system. This will help you know if there’s a maintenance problem with your system, or if your system simply isn’t providing enough power to your thermostat.
- Turn off power to your system at the fuse box or system switch. This will help prevent damage and keep you safe.
- Uninstall your Nest thermostat.
-
Connect your old thermostat. Use the picture you took of your old thermostat’s connectors during the installation of your Nest thermostat. This shows you where each wire goes.
- If your system works fine with your old thermostat re-installed, then your Nest thermostat might have a wire or power problem. You might need a common (C) wire or the Nest Power Connector installed. You might be able to connect an existing C wire or install the power connector yourself, but contact a local professional to install a new C wire and fix other issues.
- If your system doesn’t work correctly with your old thermostat and you have the same problems you had with the Nest thermostat, your system might need maintenance. Contact a local professional to find and fix problems with your system.
If you connected a Nest thermostat to a system that doesn't work with it, it will likely experience an unexpected behavior. While Nest thermostats work with most 24V systems, there are some system types and models from various manufacturers that are incompatible. Others may need to have a C wire or the Nest Power Connector to be able to work with it. Learn about systems and hardware that aren’t compatible with Nest thermostats.
Pull off your Nest thermostat’s display and find out if there’s a wire in the C connector.
If there’s already a wire in the C connector
Sometimes the wire in your Nest thermostat’s C connector isn’t properly connected. This can also be true for your system or your thermostat.
- Make sure all wires are in the right thermostat connectors. Use your thermostat wiring diagram and the picture you took of your old thermostat’s wires.
- If your thermostat won't turn on, go to our troubleshooting power issues article.
- If there's an error code or wiring report on your thermostat, go to our troubleshooting error codes article.
- If you still have issues with a wire in your thermostat’s C connector, contact a local professional. They can diagnose your system and check if your C wire is installed correctly.
If there is no wire in the C connector
Find out if there’s a C wire connected to your system’s main board. You might be able to connect it to your thermostat.
WARNING: High voltage
Be sure to turn off power to your home at the breaker, fuse box or system switch to protect yourself and help prevent damage to your system. Your furnace may be in a less accessible spot like an attic or basement.
- If you're comfortable accessing your furnace safely and you’re certain you’ve turned off power to your whole home, here are a few steps you can take.
- Check which wires are connected on your system’s control board to know if any of them are connected to the system’s C terminal. If so, you can put the other end of that wire in your Nest thermostat’s C connector to check if the strange heating or cooling behaviors stop.
- If there’s no wire in the system board’s C terminal, continue to the next steps. You can install the Nest Power Connector or have a professional HVAC technician, such as a local professional, install the power connector or a C wire for you.
- If you're not sure you can safely get to your furnace, contact a local professional in your area to help fix your problem.
The Nest Power Connector is a power accessory. It can be a simpler alternative to a C wire and also costs less. Learn how to install a Nest Power Connector.
A local professional can install the Nest Power Connector or a common wire if you don't have one already. They can find and fix issues with your system and check if your Nest thermostat is working properly.
- A pro can use a spare wire behind your thermostat as a C wire if it's not connected to your system. This is usually cheaper than running a new wire.
- A local pro may install the Nest Power Connector if you don’t have a spare wire. It can provide power to your thermostat instead of a C wire. This is often easier and costs less than running a new C wire from your system.
- Your local pro may also run a new wire from your system to your thermostat. Costs change based on how hard it is to put in the new wire. Ask your pro for an estimate.
Important: Pros are independent companies or contractors that Nest works with to help you install your Nest products. Even though they are not employed by Nest, we try to keep a network of well-rated installers for our customers to work with.