hp power supply

i have hp power supply p/n 405479-001 when i plug in the cord from the wall, the green light on the back of the P.S. turn on, and the fan on the power supply will spinn for asecond but it wont start the computer, i tested the whole components inside the Power Supply nothing short and nothing open, i know its not the main board, cause i tried to power the power supply just by it self, without connicting it to the main board, the same thing, only green light and the fan will spin for asecond does any body have any Idea what would be the reason for that?. is it achip or what? plz help thanks