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I was trying to hook up the ecobee lite in my new house today and noticed that there was a K wire (black; marked as Y wire) on my old t-stat, I did some research but found very little info on it so I decided to do some diggings myself.
I opened up my HVAC and noticed that there originally were 5 wires but somehow connected to a Honeywell hub to convert them into 4 wires. They were RC, W/OB, C, Y, G. The 4 new wires coming out of the hub were RC, W/OB, C, K.
So I assumed that the hub joint Y and G together, forming a new K wire; as online people have indicated, these are supposedly responsible for your AC unit.
Coming upstairs in the living room, I did a jumper on the K wire and had it connected to Y socket, the other end of the jumper was connected to G socket.
Voila, I gained access to my AC unit and everything was working great!
Just need to go back and solder the jumper cable for a more secure connection.
Hope you guys can benefit from my experience.
I am not responsible if this breaks anything. If you are not comfortable with doing your own jumper cable, consult ecobee and hire someone.