I'm not a tech, but I do have a little pc experience. Recently the extra side fan on my computer case died. The other fan is fine and everything else plugged into the pc is not having issues (so I'm hoping it's not my power supply). I went out and bought a fan from Best Buy, plugged it in and it doesn't work. My motherboard does not have the option of connecting through it. I have the 4 pin Molex connector.
I also tried the new fan at work, to make sure it wasn't defective. When I plugged it there, it worked perfectly.
So I have to wonder....what could be wrong with my pc other than the power supply?