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 or Nest Learning Thermostat (4th gen), 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

Troubleshoot or connect a C wire

• If you get a “The C wire isn’t getting power” error code during power test, turn off your HVAC power at the circuit breaker. Remove the thermostat display and make sure that the C wire is fully inserted then turn the power back on. If the C wire still isn’t getting power, we recommend contacting a Pro.
• 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.
  • Connect a C wire if you have an unused thermostat wire .
  • Install a Nest Power Connector if there aren’t any spare thermostat wires. 
• 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.