I have a refurbished PC that has never worked for me. When I plug in the power cable, the fan runs for maybe 3 seconds, the CD drive light flashes but the power light doesn't flash. After this, nothing happens when I press the power button. I heard that I could test the power supply by "shorting" a green wire on my 24-pin power supply connector to a black wire but there are several black wires and I'm not sure of which one to use or how to do this. Would someone be able to provide any step instructions on how to do this test? Any assistance would be greatly appreciated. Thanks in advance.