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Hello, if the system cooled before, but not now, the problem is most likely not related to the Nest thermostat.. tell me if any air is blowing out of the vents or not, and if the largest copper tube connected to the outside unit is cold or frosty, and if the unit outside is running or not.
the problem Nest encounters is that about 10% of the time it encounters a furnace/ air handler or heat pump that it was not designed nor intended to handle...
I need the brand name and model number of the furnace or air handler, and a photo of the wiring diagram glued to the inside of the blower service panel to assess the situation for you... photo must be taken up close, well lit and exceedingly well focused. You can use the paper clip icon at the right end of your tool bar to attach the photo.
I also need to know which model of Nest you have (there are 3)
Hello again, that unit is at least 15 years beyond is normal 15 year life span, and all this time with the *same fan motor... I have to call it a miracle unit at 30+ years old.
The fact that the fan is running tells us that the thermostat is working properly, and that the problem is inside the unit. It is probably not a good idea spend any money at all on repairs and should be replaced... even if you are going to sell the property that old unit will be a big deduction from the sell price.
If you wish to attempt a repair you might get lucky and the contactor that puts power to the compressor could be bad, or if you are not lucky one of the wires attached to the compressor could be burnt off.
Inspect all that, with the power off, we can go from there