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PC won't always turn on

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Hello! :) 
I have a really annoying problem with either my PSU or my motherboard.
When shut off (Or in sleep mode)during the night (It never happens if it's turn off for a short period of time), 1-2 times a week it just refuses to start (No fans spinning, nothing). The green led on the motherboard works and if it is in sleep mode, the LED on the power button flashes, but still, in both cases it won't turn on unless i turn off/on the PSU.

Also if i turn the PSU off when the PC is working, 1 in 4-5 times it also won't start when i turn it on (The second try it usually works). Another thing I noticed:
When it refuses to start and i turn off the PSU, sometimes the fans start spinning for a second on shutting the PSU down. Then, when i turn the PSU on again, it usually powers up (Without the need to press the power button again).

The PSU is fanless Seasonic 400W (It is enough for my configuration - no separate video card and energy-saving AMD. Also it's not overheating during the day). HDD, RAM and basically everything else works fine (I tried removing everything and when i unplug the PSU, it will still not always power up after that).

Could anyone help me with this problem? Is there a way to determine what really causes the issue? Thanks in advance (And sorry for my bad english). :) 

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Sounds like it could be an issue in the BIOS, what motherboard do you have and what version is your BIOS?

Could you please also list your full specifcations, as another cause of this problem would be that the PSU isn't powerful enough for the components.

JollyJay said:
Sounds like it could be an issue in the BIOS, what motherboard do you have and what version is your BIOS?

Could you please also list your full specifcations, as another cause of this problem would be that the PSU isn't powerful enough for the components.

MB is Asus M4A88T-V with bios version 0306 (Just tried resetting it, but the problem remains).
CPU: AMD Athlon II 415e
HDD: Hitachi Deskstar 500GB
RAM: 2x2GB Super T 1333 mhz
Power supply Expert
Motherboard Expert

check the troubleshooting guide in my sig - troubleshoot. sounds to me like three things

a short cos of an improper mounting of mobo to case/grounding issue
psu connection is loose so there can also be a chance of a short
loose connection to some components.
also check BIOS of your Sleep state/hybrid sleep option.

* welcome to the forums newcomer!
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Lutfij said:
check the troubleshooting guide in my sig - troubleshoot. sounds to me like three things

a short cos of an improper mounting of mobo to case/grounding issue
psu connection is loose so there can also be a chance of a short
loose connection to some components.
also check BIOS of your Sleep state/hybrid sleep option.

* welcome to the forums newcomer!

Thanks, will check it out and see what else could be wrong. :) 
Meanwhile i found and followed a tutorial on measuring the standby current, but the strange thing is it measures 5v only when the PC is turned on. When turned off, it falls to 0 (But still manages to power up most of the time?). From what i read this can't be normal? :sarcastic: 
Also i found out if it refuses to power up, sometimes it does with a 5-10 seconds delay.

Lutfij said:
you mentioned fan-less, may we have full system specs please?

Ah, sorry, forgot about the PSU. It's the X-400 Seasonic. Is it normall to have 0 volts on the purple wire when PC isn't working? And how does it even power up that way?

Lutfij said:
full system specs! :/ 

MB is Asus M4A88T-V with bios version 0306 (Just tried resetting it, but the problem remains).
CPU: AMD Athlon II 415e
HDD: Hitachi Deskstar 500GB
RAM: 2x2GB Super T 1333 mhz

That's all, no additional video card. :) 
Power supply Expert
Motherboard Expert

sorry i just noticed you posted the specs before this post...my bad!

have you gone through the troubleshooting guide? update bios and then see if anything goes.

Lutfij said:
sorry i just noticed you posted the specs before this post...my bad!

have you gone through the troubleshooting guide? update bios and then see if anything goes.

Yes, have checked everything including the RAM, but prefer not to update the bios at this time as it seems a bit risky. :) 
Power supply Expert
Motherboard Expert

you have a raid array? if not there's no risk involved just backup. if it bricks the mobo, cmos reset or remove battery for 15 secs. if all else fails, warranty.

* edit: now i know why asked for full system specs - you neglected to mention the case/ventilation

Lutfij said:
you have a raid array? if not there's no risk involved just backup. if it bricks the mobo, cmos reset or remove battery for 15 secs. if all else fails, warranty.

* edit: now i know why asked for full system specs - you neglected to mention the case/ventilation

Case is Cooler master elite 330. Cooling - Noctua (2x92mm) for the PCU, 1x rear Arctic 12mm and 2 side 80mm noiseblockers. Seems a bit too much, but i've tested without them and it stil wouldn't power up without them.

Actually after the last measuring now there is normal standby voltage when the PC's turned off, so short is now the #1 suspect. I'll see for a few more days how it behaves and will post again if the problem remains. Thanks for the help. :) 

Lutfij said:
about the short, have you removed mobo and inspected mobo standoffs?

Not yet, but I suspected two cables' jacks touching the chasis (Normally shouldn't be a problem as it should be just the plastic part touching it, but who knows). I guess this PSU is quite sensitive and has too strict protection, but i'll need some more time to say if that's it.
Power supply Expert
Motherboard Expert

hmmm, its best to give us feedback while also trying out advices. the mobo standoff is one tip mentioned in the troubleshooting guide. Should get to it rather than leaving it for another day, a short is actually worse than it seems, its cos of this safety feature your mobo is adamant and not co-operating with you, only abuse will lead it to commit suicide... your choice of options though.
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