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) 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.

Check the symptoms below for the right troubleshooting steps

  • You have one or more of the issues below, but your thermostat doesn't show an error code: 
    • Your system makes strange noises: chattering, stuttering, clicking or thumping ‒ this can happen if your system turns 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 always runs or won’t turn on
    • The system fan turns on and off repeatedly in a short period
    • Go to step 2 in this article
  • You see an error code on your thermostat or in the app:
    Your thermostat shows an error code that begins with a letter followed by a number (like N80, W4, or E73)
    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 changes temperature 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.
    Go to Nest thermostat temperature schedules to learn how schedules work and to make schedule changes.
  • Your thermostat switches to and from Eco Temperatures unexpectedly:
    • 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
      Go to Troubleshooting Home/Away Assist.
  • Other symptoms:
    If you’re experiencing a heating or cooling issue that isn’t included above, visit Google support and search for your issue.

2. Make sure all thermostat wires are fully inserted

Wiring issues usually occur when you’re first installing a Nest thermostat. But they can happen later, 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 eventually return.
    1. Turn off power to your system with the breaker, system switch, or fuse box.
    2. Wait a few minutes and turn the power back on.
    3. If the issue seems to fix itself but then returns after a short while, your system may need maintenance.

To diagnose possible system maintenance issues, 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 doesn't provide enough power to your thermostat.

  1. Turn off power to your system at the fuse box or system switch.This can prevent damage to your system and help keep you safe.
  2. Uninstall your Nest thermostat.
  3. Connect your old thermostat.
    Tip: 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, there may be a wiring or power issue with your Nest thermostat installation. You may need a common (C) wire installed. Contact a local pro to connect a 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 connected to be compatible.

Check 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. Check if you already have 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 the wiring diagram you and the picture you took of your old thermostat’s wires during installation.
  • 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 
    Turn off power to your home at the breaker, fuse box or system switch to protect yourself and help prevent damage to your system.
  2. Your furnace may be in a less accessible place like an attic or basement. If you can access your furnace safely and you’re certain you’ve turned off power to your whole home, go to step 2. If you're not sure that you can safely access your furnace, contact a local pro in your area to help troubleshoot your issue.
  3. 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 there's a wire in your system control board's C terminal, 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.
  4. If there isn’t a wire in the system board’s C terminal, continue to the next step below so you can have a professional HVAC technician, such as a local pro, install a C wire for you.

6. Have a pro install a C wire or an "add-a-wire" accessory

A local pro can install 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 works 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 your local pro may have 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.
  • A pro may also install an “add-a wire” accessory that can provide power to your thermostat instead of a C wire. Sometimes these solutions can be easier to install and cost less than running a new C wire from your system to your thermostat. Contact a local pro to install an add-a wire accessory for you.

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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