my Power Supply has only power for ~ 0.3 sec, then nothing, when I plug it in again some minutes later the same.
(Has nothing to do with my computer I bypassed it so it works without the mainboard connected.)
I connected pin 14 and 15 so the power supply should work even without a mainboard connected but here the same: power for a short time (fan spins like 3times) then no more power. It is a old computer (Athlon 900) with the original power supply (300 W) always worked fine.
If that's the case, I'd say it's either a dead power supply(they do die out you know) or something's gotten grounded out. If you have another power supply you could use, I'd suggest trying it.