My old honeywell thermostat did not have a C wire as it was ran on batteries. I did find a light blue wire when removing the old thermostat that was not being used. Looking at the furnace circuit board, there is a dark blue wire connected to C, but another group of wires that run to the thermostat with the light blue wire is not being used. My question is can I connect the light blue to the C terminal that already has the dark blue to see if I get power to my thermostat? I am guessing maybe the other set of wires are from the AC unit outside?
I tried to upload the wiring for the thermostat but it was too large. But it is a white, red, yellow, green and light blue.