That error code means Nest thermostat has an overcurrent error, typically caused by miswiring the thermostat during installation, or a worn part on the system’s control board which would require service by a professional. There are some things you can check yourself if you're comfortable doing so. Try these steps:
1. Check the system power
It sounds obvious but first, make sure that the power to your system is on. The power switch will usually be in your breaker box or fuse box, but in some homes it will look like a light switch near the heating or cooling equipment. There may be a single switch, or two switches: one for heating and one for cooling. Make sure that the power is on for both, then check your thermostat. After you check the power, go to your thermostat and check if it’s working.
2. Check the thermostat wires
Important: 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.
- Pull off the thermostat display and check that the wires are inserted correctly in the thermostat.
- Check that each wire is fully inserted into its connector and that every connector with a wire is down.
- Check that each wire has ⅜ inch (6mm) of exposed wire.
- Check that each wire is connected properly to the system board.
- Put the display back on the base and turn the power to your system back on.
- Check if your thermostat is working. If it still doesn’t work, turn off power to your system and thermostat at the breaker or fuse box again.
- Check that your system wires are connected to the right connectors. Use the wiring diagram the app gave you during setup to move the wires to the right connectors. If you don’t have the wiring diagram, refer to the picture of your thermostat wires that you took when it was installed. If you didn’t take a picture or can’t find it, follow the steps below to access your system’s control board and take a picture of the wires. Use this picture to enter the wires into the app.
- Open the Nest app. If you don’t have other products in the app, tap Add product on the home screen and follow the app instructions. If you have other products in the Nest app already, tap the Settings cog at the top right, then select Add product.
- Follow the app instructions. Make sure you choose Start installation even though you’ve already installed your thermostat.
- Follow the app instructions until the app asks you to enter your thermostat wires. Use the picture you took of your old thermostat’s wires or your control board wires as a reference.
- The app will give you a wiring diagram. Move the wires to the right connectors to match the wiring diagram. If you need to move wires, your thermostat may ask you a few questions. Make sure the wires have ⅓ to ½ inch (6mm) of copper exposed and that the wire connectors stay all the way down.
- After you have the wiring diagram, you can exit device setup.
- Turn power to your system back on at the breaker or fuse box and check if your thermostat is working.
If the issue persists after trying these steps, I'd recommend contacting a local Nest Pro or local HVAC technician in your area for more help. Hopefully the description of the issue at hand I gave at the start will be useful to them.
