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Troubleshoot Nest thermostat T error codes during a System Test

You may see a T error when running a System Test after you install your Google Nest thermostat. The most common reasons for T errors are: one or more wires are in the wrong thermostat connector, or wires aren’t fully inserted into the thermostat connectors. Here is the complete list of testing error codes:

T01 single stage fan problem

T02 single stage cooling problem

T03 two stage cooling problem

T04 forced air system problem

T05 radiant heating problem

T06 radiator heating problem

T07 baseboard electric heating problem

T08 humidifier problem

T09 dehumidifier problem

T11 two stage fan problem

T12 three stage fan problem


Troubleshooting steps for each error are very similar even though the reasons for the error can differ. Follow the steps below in order. There are specific troubleshooting steps for certain errors in some steps.

1. Run the test longer 

  • Many systems have a built-in delay so it might be 5 minutes or more before your system turns on.
  • If you’re testing a radiant system, electric baseboard heating, or a system with radiators it typically takes a while before you can feel the floor baseboard or radiators getting warm. Try waiting up to 30 minutes to see if they warm up.
  • The System Test will run for 35 minutes before automatically stopping. Select Keep Testing on your thermostat if needed or restart testing by going to Settings>Equipment on your thermostat. Select Continue and then Test.

2. Make sure your system has power 

  • Check that the breaker switch or fuse that controls power to your system is set to on. System power should be turned off during thermostat installation to help prevent blowing a fuse but it should be turned on for testing.
  • Also check the switches for other system components such as fans.  They may have a separate switch which also needs to be set to on.

3. Check that you've wired the thermostat correctly 

CAUTION: It’s important to protect yourself your thermostat and your system. Turn off power to your system and thermostat at the breaker or fuse box before exposing or adjusting any wiring.

1. Pull of the Nest thermostat display.

2. Make sure that your system wires are connected to your thermostat correctly. Use the wiring diagram the Nest app gave you during setup.

  • If you don’t have a wiring diagram
    • Refer to the picture of your thermostat wires that you were instructed to take by the Nest app during setup.
    • If you didn’t take a picture or can’t find it, follow these instructions, and take a picture of your system’s wires on the control board. Use this picture to enter the wires into the Nest app

      1. Open the Nest app Nest app.
      2. Tap Add Plus icon.
      • If you already have a Nest product in the app, tap Settings Nest settings icon in the top right corner of the app home screen. Then select Add product and pick the thermostat model you have to install. 
      • Select Start installation (even if you have already installed your thermostat).
      • Follow the app instructions until you get to the wiring diagram step. Use the picture you took of your system’s control board to enter your thermostat wires into the app.

3. Make sure you’re turned off power to your system at the breaker or fuse box to help prevent damage.

4. Move the wires to different connectors to match the wiring diagram. If you need to move wires, your thermostat will ask you a few questions.

5. If the wires are in the right Nest thermostat connectors:

  • Check that each wire is fully inserted into its connector
    and that every connector with a wire is down as shown.
  • Check that each wire has ⅜ inch (6mm) of exposed wire.
thermostat connector button down


6. Turn power to your system back on at the breaker or fuse box.

7. Re-test your system. On your thermostat go to Settings>Equipment. Then select Continue and then Test.

4. Steps for specific system types 

Check for each case below then restart the System Test if needed.

  • If your system has radiators (T06 error) Check that the radiator valves are fully open so that hot water can circulate to the radiators.
  • If you have an electric baseboard system (T07 error) Most baseboard systems are high voltage (120 V) and are incompatible with Nest thermostats. 24V baseboard systems are much less common so double check your system voltage. If it’s 120 V disconnect the Nest thermostat and reinstall your old one.
  • If you have a heat pump (there’s a wire in the O|B connector) If there’s a wire (usually orange) in your Nest thermostat’s O|B or * O|B connector you have a heat pump. Heat pump systems can heat when they’re supposed to cool (or cool when they’re supposed to heat) if the O|B wire isn’t configured right during thermostat setup. See this article for specific steps to change the setting.
  • If you have a dehumidifier or humidifier (T09 or T08 error) We strongly recommend professional installation for systems with dehumidifiers or humidifiers because of setup complexity. Nest thermostats have one connector ( * or * O|B) for dehumidifiers or humidifiers. But these components often have two wires so a relay must be installed to convert the two wires into one for Nest compatibility. In addition the dehumidifier or humidifier must be configured correctly in the thermostat’s Pro Setup menu.

5. Test your old thermostat again and contact a pro 

Nest thermostats are designed for DIY installation but some systems are complex and difficult to set up correctly. In addition there may be an issue with your system. For instance sometimes a system fuse gets blown if thermostat wires make contact with each other during installation. If you’ve followed the above steps contact a local professional to help you get your Nest thermostat connected to your system and set up correctly.

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