Troubleshoot an overcurrent error on your Nest thermostat

Overcurrent errors that begin with the letter E are typically caused by miswiring the thermostat during installation, or a worn part on the system’s control board which will require service by a professional.

Google Nest thermostats can help detect and diagnose issues with your system. When your thermostat shows you an overcurrent error it’s because it has detected multiple power spikes. You may not have noticed anything was wrong before because there was no way your old thermostat could let you know.

Common causes of overcurrent errors

If you see an overcurrent or “E” error, the electrical current on one or more wires connected to your Nest thermostat has spiked multiple times. Nest thermostats have an internal breaker to help prevent damage, and you will only see an overcurrent error if the breaker has been tripped 10 times or more. So an overcurrent error is likely caused by a recurring problem and won’t go away by itself. Here are some of the most common causes:

  • There is a wire in the wrong Nest thermostat connector.
  • There is more than one wire in a single Nest thermostat connector.
  • A relay or transformer on your system’s control board is worn or has failed.
  • There were recent issues or upgrades made to your home’s electrical system.
  • There were recent repairs or service done to your system.

Troubleshoot an overcurrent error

1. Look up which wire to check

See the table below for the wire connector you should check on your Nest thermostat. For instance, if your thermostat shows you an E57 error, you’ll need to check the Y2 wire.

Error Code

Wire to check

E52, E102 or E182

W1

E53, E103 or E183

Y1

E54, E104 or E184

G

E55 or E105

O/B

E56 or E106

W2/Aux

E57 or E107

Y2

E58, E108 or E188

* or */OB

 

2. Check your thermostat's wiring

Important: Be sure to turn off power to your system at the breaker, fuse box, or system switch to protect yourself and help prevent damage to your system.

  1. Pull off your Nest thermostat’s display so you can check the wire in the connector you got from the table above.
  2. Check that there is only one wire in the connector (no jumper wires or other wires).
  3. Make sure that the right wire is in the connector. Use one of the following two things to check against:
    • The wiring diagram the Nest app gave you during setup.
    • The wiring diagram you got from our online compatibility checker.
  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. Important: If you have wires in the Rc and Rh connectors, double check that they weren’t accidentally switched during installation. Swap them if they were.

If you don’t have a thermostat wiring diagram

  • Use the picture of your old thermostat wires that you were instructed to take by the app during setup, and enter them into our online compatibility checker.
  • If you didn’t take a picture of your thermostat wires or can’t find it, follow these instructions to take a picture of your system’s wires on the control board. Then start thermostat installation with the app again and re-enter the wires when the app asks you to.

3. Contact a Nest Pro to check your system

If the thermostat wiring is correct, the cause is likely a problem with a damaged relay or transformer on your system’s control board, not your Nest thermostat. Contact a local Nest Pro to service your system.

How to help your Nest Pro find and fix the problem

You should give your Nest Pro the information below so they can troubleshoot more effectively:

  • Tell them which wire is causing the problem (see the error codes and their related wires in step 1)
  • Let them know if there were any recent repairs to your system (Did you see the error code pop up soon after service?) It’s possible that something wasn’t connected properly or came loose.
  • Let them know that the Nest thermostat senses very fast and small voltage changes (multiple per second). So if they check the wire with a handheld multimeter it probably won’t show any power spikes.
  • Ask them to check and replace the relay of the wire that’s giving you the error.
  • Also ask them to check the transformers on the system board. They should replace any that are worn or damaged.

Note: If the Nest Pro can’t find any worn parts or wire issues with your system, in some rare cases, recent issues with your home’s electrical system, or upgrades made to it could have caused an overcurrent error.

4. You may have an incompatible system

Some systems that are working normally deliver electrical current to the thermostat wires in an unconventional way so that the Nest thermostat’s internal breaker gets continually tripped so it can protect itself against damage from too much voltage. If this is the case, your system isn’t Nest compatible and you should reinstall your old thermostat.

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