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?

Furnace: Furnace

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.

2 Answers 2


Yes, you are right on track here

Simply connecting the light blue wire to the C terminal on your furnace control board (with the furnace turned off at the service switch or breaker, of course) will get you a C wire for your new thermostat. You are also correct that the cable with the yellow and dark blue wires connected in it goes off to your air conditioner (outdoor unit); it is what carries the signal to the air conditioner that calls for it to run the compressor.

  • You certainly can
    – Joe Fala
    Apr 1, 2019 at 2:46

Please test before connecting two wires together one might be live the other ground. result bang.Steve uk engineer.

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