Nest thermostat only works when swapping with other Nest-controlled unit

[yitbos1]

I have two HVAC units, both controlled by Nest learning thermostats.  They both have worked great for the past several years that I have had them.  Nest 1 is connected to a system without a C-wire without issues and Nest 2 is connected to a system with a C-wire.

I woke up to Nest 1 not getting power.  I traced the issue (that I thought at the time) to a tripped GFCI that the condensate pump is plugged into, causing a condensate backup, shutting off the unit.  This has happened once before several months ago and everything continued operation just fine after resetting.  This time, the thermostat still does not power up. 

I wondered if it was a faulty Nest and tested it by swapping Nest 1 for Nest 2.  Weird thing is they both work fine when swapped.  Nest 1 will not work in its original location.  🤔

What could the issue be?  Did I have a power surge that - while tripping the GFCI - also did damage to the Nest?  Does the fact that the problem Nest was attached to a system without a C-wire mean anything, especially since it worked fine for several years that way?

[CoolingWizard · Accepted solution]

@yitbos1 , a GFCI is not a surge suppressor. Why did he text is an electrical flow outside of the balance of the line, the neutral and the ground. if your GFCI has tripped there is an electrical short someplace in that pump. Or whatever devices connected to it. 
Since you say a GFCI trip cause the Nest Thermostat to loose power to charge its battery, then your air handler/furnace must be powered by the same GFCI that the condensate pump is connected to, correct?

If there is a ground fault imbalance problem in your air handler, there can be damage to the control board or the step-down transformer. Test the transformer output on the low side for 24VAC. It should be more than 29VAC or less than 19VAC. 

AC Cooling Wizard