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Troubleshoot strange heating or cooling behavior

Sometimes heating or cooling systems can behave strangely. For instance, you may notice your system is always on, it won’t turn on, it turns off and on repeatedly, or it makes strange noises. If so, there may be a power issue. In some cases, you may need to install a C wire (also called a common wire) or the Nest Power Connector to help deliver consistent power to your thermostat.

There are a number of other reasons why your heating and cooling system might be doing something unexpected.

1. Find the right troubleshooting steps

  • Your system is acting strangely: If you see one of the issues listed below without seeing an error code on your Nest thermostat, continue to step 2 to troubleshoot.
    • Your system is making strange noises: 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
    • You see a Delayed or Starts in message on your Nest thermostat’s screen (for example: “Heating is delayed for 2:30 mins”)
    • The system fan is always running or won’t turn on
    • The system fan turns on and off repeatedly in a short period
  • You see an error code on your thermostat or in the app: If your thermostat shows an error code that begins with a letter followed by a number (like N80, W4, or E73), please go to Troubleshooting Nest thermostat error codes.
  • Your system cools when it’s supposed to heat, or heats when you want cooling: 
    Go to Troubleshoot when your heat pump heats when it’s supposed to cool.
  • Your thermostat temperature changes unexpectedly: If your thermostat seems to have a mind of its own and sets itself to a temperature that you don’t want, you may have an issue with the temperature schedule. Please see the article about Nest thermostat temperature schedules to learn how it works and how to make changes if needed.
  • Your thermostat switches to and from Eco Temperature unexpectedly: If you see one or more of the symptoms listed below, please go to Troubleshooting automatic home and device switching
    • Your thermostat switches to Eco even though you’re home.
    • Your thermostat stays in Eco even if you’re home.
    • Your thermostat takes a long time to switch to and from Eco.
  • Other symptoms: If you’re experiencing a heating or cooling issue that isn’t included above, visit Google support and try searching our support site for your issue.
2. Make sure all thermostat wires are fully inserted

If you checked the symptom lists in step 1 above and your symptoms match, here are some troubleshooting steps you can take.

Wiring issues usually occur when you’re first installing a Nest thermostat. However, they can surface later as well, but usually for different reasons.

  1. Turn off power to your system at the fuse box or system switch. This will help prevent damage to your system and help keep you safe.
  2. Pull off the Nest thermostat display.
  3. Check that all connectors that have wires are down (see picture). 
  4. If needed, re-insert wires so that the connector button stays fully pressed down. If the button isn’t down, your Nest thermostat won’t know there’s a wire in the connector. 
  5. You may need to re-strip and straighten the wire ends so that they can be fully inserted into the Nest base’s connectors.
Terminal button
3. Check that your system doesn't need maintenance

Here are some common symptoms that indicate that your system may 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. 
    • Note: 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 listed above in this article’s introduction eventually return.
        
  1. Test by turning power to your system off at the breaker switch.
  2. Wait a few minutes and turn the power back on.
  3. If the issue seemed to fix itself but then returns after a short while, your system may need maintenance.

To diagnose possible system maintenance issues yourself, you can install your old thermostat and test your system. This will help you tell if there’s a maintenance problem with your system, or if your system simply isn’t providing enough power to your thermostat.

  1. Turn off power to your system at the fuse box or system switch.This will help prevent damage to your system and help keep you safe.
  2. Uninstall your Nest thermostat.
  3. Connect your old thermostat. Use the picture you took of your old thermostat’s connectors during the installation of your Nest thermostat to help you know where each wire should go on your old thermostat.
    • If your system works correctly with your old thermostat re-installed, then there may be a wiring or power issue with your Nest thermostat installation. You may need a common (C) wire or the Nest Power Connector installed. You may be able to connect an existing C wire or install the power connector yourself, but contact a local pro to install a new C wire and troubleshoot any other issues.
    • If your system doesn’t work correctly with your old thermostat and you see the same issues you had with the Nest thermostat, your system may need maintenance. Contact a local pro to diagnose and fix issues with your system.
4. Check that you have a compatible system

If you connected a Nest thermostat to an incompatible system you will likely experience unexpected behavior. While Nest thermostats work with most 24V systems, there are some system types and some specific models from various manufacturers that are incompatible. Others may need to have a C wire or the Nest Power Connector connected to be compatible. See the following article for a list of system types that aren’t Nest compatible.

Systems and hardware that aren’t compatible with Nest thermostats

5. Fix or connect a C wire

Pull off your Nest thermostat’s display and see 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 to your system or your thermostat.

  • Double check that 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, please see our troubleshooting power issues article.
  • If you see an error code or wiring report on your thermostat, please go to our troubleshooting error codes article.
  • If you’re still having issues with a wire in your thermostat’s C connector, contact a local pro to diagnose your system and check that your C wire is installed correctly.

If there isn’t a wire in the C connector

See if there’s a C wire connected to your system’s board that you can connect to your thermostat.

  1. 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 place 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.
    • If you're not sure that you can safely access your furnace, contact a local pro in your area to help troubleshoot your issue.
  2. Look at which wires are connected on your system’s control board to see 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 see if the strange heating or cooling behaviors stop.
  3. If there isn’t a 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 pro, install the power connector or a C wire for you.
6. Install the Nest Power Connector
The Nest Power Connector is a power accessory that may be a simpler and more affordable alternative to a C wire. To learn how to install it, go to The Nest Power Connector.
7. Have a pro install the Nest Power Connector or a C wire

local pro can install the Nest Power Connector or a common wire if you don't have one already. They can also diagnose and fix issues with your system and check that your Nest thermostat is working properly.

  • If you have a spare wire behind your thermostat that’s not connected to your system, your local pro may be able to use it as a C wire, which is usually cheaper than running a new wire to your thermostat.
  • If you don’t have a spare wire, a local pro may be able to install the Nest Power Connector. It can provide power to your thermostat instead of a C wire. Typically, this is easier and less expensive to install than running a new C wire from your system to your thermostat.
  • Your local pro may also be able to run a new wire from your system to your thermostat. Costs can vary depending on how difficult running the new wire will be. Ask your pro for an estimate.

Contact a local pro

Note: Pros are independent companies or contractors that Nest works with to help you install your Nest products. Although they are not employed by Nest, we try to maintain a network of well-rated installers for our customers to work with.

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