[barrysg1]

I've recently add an AC system to my existing forced air natural gas furnace. This system is 30-years-old and still works like it should. About 5-years-ago, I added a Nest learning thermostat and have never had any issues, it has always performed as designed.
What is happening now, is the Nest does not register the installed AC system, (the yellow wire, or the fan, the green wire) as you will see by the photos. I tested the continuity of both the yellow and the green wire by connecting a 9-volt battery to the disconnected wires at the control board, then measured the voltage present at the disconnected wires at the thermostat, it was 9.01 VDC showing continuity. Then I added another power wire to terminal marked "Rc" by connecting the brown wire as seen in the photos. I've also tested each individual circuit by disconnecting all wires from the Nest and connecting the heat circuit manually, it worked, the fan, it worked, the AC wire, it worked, then the fan wire, it also worked when connected to the 24VAC, "red wire."
However, when I connected the "yellow wire," and the "green fan wire," at the same time, it will not turn on both at the same time. I don't know if this is an indication of anything, that's just my observation.

I've attached photos of my system if anyone has any advice as to what course of action to continue.

Best regards,

 

[barrysg1]

After a considerable amount of testing at the furnace with a set of jumpers, then the temporary installation of a alternate thermostat, I arrived at the conclusion that the root cause of this was the result of a defective Nest terminal base, not recognizing Y1, it would register Y2 as Y1 but I had no second stage cooling system.