According to the documentation that I have, Pin (socket) 3 is for the Continuous Fuel Injection System (CFI) and as you indicated earlier addresses the same DM as the built-in LED for Cal versions. I suspected this earlier since the flashing light on the home-made reader flashed in sync with the built-in LED while my yellow wire was in this socket.

I now have 2 codes present on that socket: 17 "Oxygen sensor is shorted to positive or ground" , and as of yesterday 18 "Current to idle control valve is illogical".

The code 17 is the same one that I had about 2 years ago. Replaced Oxy sensor, tested all wiring, then someone on this group suggested checking the Duty Cycle. Made adjustments to that and the code has been gone since then. About 1 month ago my CE light came on and Code 17 returned. That is what got me motivated to build the code reader and get everything tested and fixed.

Code 18 is new since I erased the memory yesterday and started having the idle trouble.