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Identify hardware error in non-bootable desktop


I have a desktop that has boot problems that can only be from an hardware component.

The desktop will shutdown at apparently random moments, it can be immediatly before the screen even turns on, it can be anywhere during the BIOS/OS boot process or even when the OS is running and fully operational.

It might just be me, but it seems that on the occasions where the system is able to fully boot into to the OS, the chance of turning off automatically is reduced since it is able to stay on for hours/days.

The power off behaviour seems to be pretty random, and keeping the system off with the power cable removed for several hours to discharge does not seem to help much. If I insist on turning on the system, in the same minute it can turn off 3 times immediatly, 2 after a few seconds, and finally fully boot into to the OS.

I suspect this behaviour might be from a problem with the PSU, CPU, Mobo or RAM but I would really love to avoid the diagnosis by trial removal and substitution of components since I don't have similar spares. Is anyone able to assess with some certainty what component could be causing this behaviour?

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Scott C
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Run memtest in order to ensure RAM is working fine.
CPU: check cooling and check there's no overclocking (does anyone overclock nowadays?).
PSU: remove power from unnecessary peripherals (CD/DVD-ROMs?).
Remove any add-on cards that aren't strictly need (sound card?)

There is nothing that tells you which hardware doesn't work as expected. You must work on it using the suggestions above. to test your memory

and the hard drive diagnostic from the HD manufacturer.
How old is motherboard? May be there are bad capacitors? Inspect motherboard visually at first. If it is new and shutdown occurs without blue screen - I vote for PSU.
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Thanks for the quick feedback guys.

memtest: thought about it, but given how unstable the system is and how hard it is to even boot, I don't believe I will even be able to run it till the end.

Motherboard: about 3 year old motherboard, never overclocked.

There are no bluescreens, it just shuts down completely at random.

I don't believe it is an hardrive problem, I don't think it would turn off the system during a BIOS boot.

I was trying to avoid it, I hoped the symptons could be a specific giveaway to identify the component, but I see now that I will have to remove all components, dust off everything, and try to boot the system by adding components one by one. I will also take the opportunity to change the thermal paste on the CPU.
Consider the option the PSU might be undersized. When consumed power reaches a certain level the power simply goes down.
Good point.  If this is the case disconnecting the optical drive(s) might be enough to move the tipping point back.
I would suspect motherboard or PSU.

You can test the PSU with a multimeter, if you have one. Boot it up and put the negative lead in any ground, then move the positive lead from each color to the next.

Charges should be 3.3v, 5v, 12v. If any are fluctuating wildly (Commonly 3.3v) than you can suspect the PSU.
Do you mean capacitors?

If so, not very easy to change out unless you are a wizard with a soldering iron.
use the minimum setup as described in my troubleshooting guide :                  (Short-overview-of-how-to-troubleshoot-bad-hardware-when-a-pc-does-not-post)

if the disk is suspected, use this :
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Surprisingly this seemed to have been enough. I removed all components and cleaned every slot with compressed air and it booted immediatly and perfectly ever since.

Ran memory checks for over 12 hours straight and very demanding apps and it just works flawlesly. I had seen dust create some random blue screens, probably due to accumulated static or something similar, but complete random shutdowns was a first. Thanks to all for the support, and to ScottCha in particular for nailing it.

Thanks and glad to help.