Computer turns on but turns off almost instantly

[Josildo]

Troubleshooting starter

1. Overview of the problem
   My computer turns on but turns off almost instantly afterwards.
   
   
2. Full description of the problem and symptoms
   Okay, here is the deal. I built this PC a few months ago, assembled it fine and everything is running perfectly. I have honestly never had any issues at all. Well earlier tonight I went out with some friends, came home and when I went to turn on my computer, it switches on almost instantly until it switches itself back off. I will explain it in as much detail as I can:
   
   When I turn it on, I see it switch on for a split moment and then it just switches off. None of the fans boot up etc, but I see it turn on from the LED on my front case fan coming on then going off. The motherboard LED is on and is green. But when I try to turn it on afterwards, it just doesn't turn on at all. I've unplugged the power supply and left it out for a minute and plugged it back in, but I get the same problem as described up until this point again, involving having to unplug and try again to no avail. This is my problem and I am absolutely baffled.
   
   I apologise if this isn't descriptive enough but that is essentially the issue.
   
   
   
3. Did it work normally at one time, or has the problem always existed?
   My computer had been working perfectly fine up until today. I never had any issues with it at all. I simply turned it off normally, went out for a few hours, come back home and now I have this issue.
   
   
   
4. Is the problem consistent and repeatable, or entirely random, or semi-random?
   This problem is completely random.
   
   
   
5. I already tried these steps:
   Here is what I have tried:
   a) unplugging the power supply for a while, and plugging it back in
   b) Checked the power button to see if it was the issue
   c) checking all power connections to the mobo/the power button connections/led connections
   b) resetting bios on mobo
   
   
   
6. My software:
   • Windows XP SP3
     
     
     
     
   • My hardware
     • Asus P5Q (1303 bios i think? Can't remember off the top of my head)
     • E8400 (OCed)
     • HIS 4870x2 2G
     • G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500)
     • Antec 850W signature series (TPQ 850)
     
     

 

[mpilchfamily]

I seam to recall there being some issues between the Asus P5Q boards and Antec's TPQ 850. Had something to do with the power good signal between the PSU and motherboard are slightly out of faze causing this kind of power on power off problem. You may want to contact Antec about it and see what they have to say. But you may end up needing a different PSU.