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Learn about the common or C wire

Nest thermostats are designed to use very little power, and in many cases they can use your system’s heating and cooling wires to get enough consistent power. But sometimes, you may need to connect a common wire (C wire) or power accessory, like the Nest Power Connector, to make your system compatible.

Unlike other wires connected to your thermostat, a C wire doesn’t control heating or cooling functions. It only provides constant power to your thermostat.

Learn how to identify a thermostat wire.

If you’re installing a Nest Thermostat, the Google Home app will often tell you during setup if you need to connect a C wire. It may also test to check if the C wire delivers enough power to your Nest thermostat.

With a Nest Thermostat E or Nest Learning Thermostat, you’ll need to install a C wire if the thermostat has power issues or its battery often drains.

How to tell if your Nest thermostat needs a C wire

Before you buy a Nest thermostat

You can check if you need a C wire before you purchase a Nest thermostat at

During setup (Nest Thermostat only)

This section only applies to Nest Thermostats. If you have a Nest Thermostat E or Nest Learning Thermostat, check the section “After installation and setup.”

During initial setup, the app will often tell you if you should connect a C wire to your thermostat. If your old thermostat had a C wire connected, the app will typically tell you to connect a C wire to your Nest thermostat.

Important: Some systems, including heating-only, cooling-only, zone-controlled, and heat pump systems, require a C wire or a compatible power accessory, like the Nest Power Connector. The app will tell you if you need this.

How to tell which Nest thermostat you have

After installation and setup

In some cases, you may experience problems after installing your Nest thermostat that can be fixed by connecting a Nest Power Connector or a C wire.

If your system can’t deliver consistent power to your thermostat to keep its battery charged or correctly control heating and cooling, you may experience one or more of these symptoms: 

  • Reduced thermostat battery life
  • Thermostat display won’t turn on
  • Thermostat motion sensing is disabled
  • Your thermostat occasionally disconnects from Wi-Fi
  • Your system unexpectedly turns on or off
  • Your system is making strange noises: chattering, stuttering, clicking or thumping
  • Heating or cooling is always on, and won’t turn off
  • Heating or cooling repeatedly turns on and off in a short period
  • A “Delayed” orStarts in” message often appears 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 get an E1, E297, N260 or N261 error code

Go to the section below for help fixing these power related issues.

Troubleshoot or connect a C wire

  • On your thermostat, check for codes that can help you understand what's wrong. If you find a help code, follow the troubleshooting steps in Troubleshoot Nest thermostat help codes.
  • If you’re not sure what’s wrong or don’t feel comfortable fixing your system, the best solution for power issues is to contact a Nest Pro in your area. They can diagnose and help fix any possible problems with your system and thermostat installation. If needed, they can also connect a C wire or a C wire accessory, such as a Nest Power Connector to your Nest thermostat.
  • If you’re comfortable troubleshooting your system and take necessary precautions, you might be able to fix C wire issues yourself. Always make sure to turn the power to your HVAC system off at the breaker before attempting any fixes or installations.
  • If you connected a wire to your thermostat’s C terminal and it caused your system to malfunction, you’ll need to check a few things:
    •  Make sure the wire is fully inserted into the thermostat connector.
    • Turn off power to your system and check that the wire is connected to the C terminal on your system’s control board. 
    • Turn power to your system back on and check that the issue is fixed. If not, contact a local Nest Pro for help.


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