Question
When the controller is plugged in and set to "RUN", the display reads "NO AC".  I have checked the voltage where the transformer wires connect to the internal module. (Green wire to GND and 2 Yellow wires each to an "AC" terminal.)  0 volts between GND and first yellow wire, 24 volts between GND and second yellow wire.  This is the same as on another Pro C controller that is working fine.  Any ideas why this controller says "NO AC"?

Answer
Hey Larry, I hope this helps. Normally when "NO AC" is displayed, it means the controller is not receiving power. You have obviously checked your voltage at the controller and you are getting power. By the way the correct way to check if the transformer is good is to touch both yellow wires with the volt meter, but you are still fine in the way you did it. I would suspect that your circuit board has a bad connection somewhere, or a bad wire. You're better off just buying a new controller. Thanks and have a great day.