Getting N261 Error after battery change - C Wire is present - never issue previously

[midcoasting15]

This device had been working fine for 4 years - recently changed battery and now getting N261 error message. C wire is attached - all other Nest thermostats in home have same config and are working, with no error messages. Any ideas? 

[CoolingWizard · Accepted solution]

@midcoasting15 , 

as I am sure, you are aware, the N261 error indicates that there’s low power or current flowing on the C wire. as long as you did not accidentally move or change any of the other wires, all should be good. Unless of course your HVAC power transformer has not failed. 
please get a multimeter or Volt Ohm Meter and check the AC power between the R wire and the C wire. You should read at approximately 24 Volts no more than 28 V and no less than 19 Volts. 

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The AC Cooling Wizard