Contactor is located in outside ac unit. This should be checked, but is
not only possible cause. Fuse is likely mounted on control board in
inside air handler unit.
Could be partial short in wiring, or could be Thermostat. You should see
24v. Check voltage at various points along y1 wire, starting at
Thermostat itself, disconnecting wire along those same points at
available connections.
Error in my case was caused by a faulty contactor in outside ac unit.
This acts like a power relay to switch on the outside unit. Nest is
sensitive, even to partial short in this component, which may not blow
fuse in inside air handling unit. Note th...