Nest 3rd Gen Thermostat showing E74 error. Not turning on furnace.

[maximum5869]

I have a 3rd generation thermostat. It has worked fine for years. A few days ago it completely shut off, like it is dead. We have tried to charge it and it will turn on enough to give us an E74 error. We can't find any issues with the wiring and the wiring to our furnace is fine. The only issue we are having right now is the ignitor for our furnace keeps blowing out. We have gone three days without heat in Pennsylvania winter because of this. How can we fix this so it's working? 

[CoolingWizard]

@maximum5869 ,

An E74 error means that the Furnace is not sending 24V to the thermostat on the Rh wire.  Check your power at your furnace.  Using a Volt Ohm Multimeter, check the Alternating Current power coming into the furnace and the transformer, and the power leaving the transformer.  

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AC Cooling Wizard