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Two identical systems in the home. Single stage heat, 2 stage cooling.   Both were functioning fine with Nest installed for two months.  Last Friday, I got an E103 error on the app as soon as cooling turns on.  Tried a new thermostat - same error.  Have rechecked the wiring - all ok.  Regular Honeywell thermostat works the system fine.  Have measured control transformer outputs - 27.8V... C to Y1 - 27.8V as well.

Upstairs unit wired exactly the same and works without issues.  

Not sure at this point what else I can check.  Talked with Nest support and they indicated that they think it’s an over current spike - but I can’t figure out where that would be coming from.  Error is quite consistent and doesn’t clear itself.

Any suggestions?
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Hi Jim,

Thanks for bringing this up. This error code means that there's an over current spike on the Y1 wire. To clear the error, we can reboot the thermostat by pressing and holding it for 20 seconds. When the thermostat boots up, it should return to the home screen and continue normal operation. Also, we can try turning the power to the HVAC system off at the breakers and turning it back on and check if you'll get the error code again.

Let us know how it goes.


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Resetting the thermostat doesn’t help.  It immediately gives the same error as soon as cooling is called for.  My bigger question is where would the overcurrent be coming from?  Control transformer and outputs from the circuit board are normal.
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I have 4 Nest thermostats in my building. I have 2 at home. All have functioned fine for years. (almost 3 years) This morning one of my thermostats at home suddenly has not enough power to run wifi. Hmmm.
When i get to work the thermostat in my office has the e103 error. The one upstairs in the building is offline.
Three out of six thermostats!! In the span of a few hours?!
I rebooted the thermostat in my office and found the problem.

It's the big "G" that comes up when it reboots.
Sell it back to Nest...
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Thanks for reaching out. 
Since your 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. 
We suggest contacting a local Nest Pro tech in your area to service your system. Important: 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.
Let us know how it goes.
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