Yellow wire at fault; now what?

[Vincent_Panther]

My AC quit working --- in the peak summer heat, naturally --- and my 1st-Gen Nest Learning Thermostat began blinking both red and green lights. Following the steps recommended on these forum threads, I recharged the battery, checked all the wires and connections, then received the same alternating messages to remove and reattach the thermostat that others have described.  I went wire by wire and found that disconnecting the yellow (cooling) wire in the thermostat allows it to work normally again, though obviously without functional AC.   Oh, and the thermostat indicates there's no power to the red (Rh) wire.  So could the wiring or the base be the issue?  Or any other suggestions?  Thanks in advance for the help.

[Patrick_Caezza]

@Vincent_Panther 

Sounds like the problem is with the HVAC unit itself.

Check to see if it has power and if none of the safety switches have been tripped, also get that the control board fuse hasn't blown.

[Vincent_Panther]

Thanks to your feedback, @Patrick_Caezza, I arranged for a local contractor to check out the system. The technician was able to run each component (fan only, AC, and furnace) separately when bypassing the thermostat, confirming it was the culprit, and also confirming that all my troubleshooting was on the correct path. Now, $350 ($100 for the service call + $250 at the hardware store) and some elbow grease later, I have the latest top-rated smart thermostat on my wall (Nest products performed surprisingly poorly on that list, btw), a new app on my phone to control it, and a comfortably cool house.  Rest in peace, 1st-Gen Nest Learning Thermostat; you did well for nearly 12 years. Now, does anybody know if & how I can recycle the old unit??