Computer Refuses to boot. LED for fan switches on for a second than off.

I've tried disconnecting all devices except mobo/cpu/power just to get into bios, but still encountering same issue. PC seemingly won't turn on for more than .1s then turns back off. It is not new, or specific model (custom built) but has been working flawlessly for almost a year until now.

Also the LED on back of case is constantly on meaning there is juice getting to the box. And sometimes hitting power switch has no effect and it wont turn on for even .1s.

Any ideas/suggestions?
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Wandering99Author Commented:
CPU is intel Duo Core, 2 gb ram, Asus Gbit Motherboard. Also should mention unplugged power connections and re-plugged just for kicks no difference.
It sounds like a power supply issue. You may need to replace it. Since it is only just over a year old the PSU may be under warranty and you can get it replaced.
I hope this helps. Good luck.
Brian PringleSystems Analyst II, SCM, ERPCommented:
Take the computer to a local computer shop and ask them if they have a "POST Code" tester.  It is a device that plugs into a PCI slot and will read all of the POST codes.  Also, have them hook up the power supply to a PSU tester.  As andrew_aj1 said, the PSU could be bad.

Between these two things, you should be able to narrow down the problem.
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I've seen this before, it drove me nuts until I realized I had a good power supply but a bad power switch behind the power-on button.  Try this:

1. Open the case.
2. Disconnect the power-on (white & green) wire from the power-on jumpers on the motherboard.
3. Take a jumper (or whatever method you like) and jump the power-on pins by quickly putting on then removing the jumper.

If the computer boots normally (and stays on) - you can verify that the switch is the problem. If so, get new switch (or bypass it with your own).
>>  disconnecting all devices except mobo/cpu/power just to get into bios   <<   you left 1 ram stick i hope - you need that !
here my recepy for these cases :
i suggest testing with the minimum setup - disconnect also all peripherals and network cables :
motherboard + cpu + 1 ram stick, video card, power supply
on boot, do you have a display?
if NO it is one of the connected, swap ram, Power supply, video card - leaving only motherboard and cpu
if Yes, add devices till the problem shows

you can also check the motherboard for bad capacitors as shown here :
First thing I would try is to "remove" the power cable, then hold the power button in for a couple of seconds to clear any residual power from the system.  

Second I would probably try a different memory chip if you have one available or re-seat the current memory.

Third I would probably clear cmos with jumper (usually 3 pin jumper you move).  If you can give me pc type/motherboard type I can probably give you coordinates of the jumper but it's usually near the battery on the motherboard.   You will have to reset your bios settings if anything from default after this step.

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Wandering99Author Commented:
After about 50 tries it finally booted normally and all seemed fine. However i turned the pc off via shutdown. And tried to start manually via power button, and get the same problem. Took another 50 tries to get it back up. Reading comments now, going to try method related directly to power button first and go from there will post results.
Wandering99Author Commented:
Booting/rebooting appears to work fine after 1 successful boot; as long as i don't disconnect power from the back of the pc

If the power cord directly connected to back of pc/power supply is disconnected I have to go through 50 tries. But as soon as i get 1 successful attempt everything works fine.(until power cord is disconnected again).
PS or mobo probably - did you try my routine to troubleshoot ?
Wandering99Author Commented:
will try later today your method and the power switch 1
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