My primary desktop computer has stopped working entirely. Nothing happens when I press the power button. No noise, no motion, no blinking LEDs, absolutely no response.

However, the power LED on the motherboard lights up when the power supply is plugged in to AC power.

I'm looking for advice on a testing process to determine which is the problematic component. I don't have any spare computer parts, so I can't replace parts individually in an elimination process. I could buy some cheap temporary components for the test, but I'd like to consider this a last resort.


  1. From the fact that the motherboard LED is on, can I conclude the power supply is providing sufficient power for the motherboard to start?
  2. If the CPU was damaged, could I expect the motherboard to at least power up the BIOS?

The motherboard is an Asus P5KC


See if the power supply works: Unplug all cables from the PSU and take the plug (P1) that goes into the mainboard.

P1 connector

Now, one of the wires is named PS_ON, that's usually a green wire, but check with your manual, and on the internet. You need to short that wire against any COM wire. If the PSU starts its colling fan and powers the other current wires (+/-3.3V or +/-5V or +/-12V) your PSU is working.

Once that's settled, remove the CPU, RAM and all other peripherial (PCI cards, drives etc) and start the mainboard. If it powers on, you should insert the CPU and RAM and see if it still powers on. Next is graphics card. Then drives, and finally PCI peripheria.

  • Are you sure it's wise to unplug everything? I'm 100% sure that PC power supplies have minimum load needed for them to work correctly.
    – AndrejaKo
    Sep 23 '10 at 1:43
  • Absolutely. In order to put some load onto the unit, you can stick a 200kOhm resistor into a 5V line and into COM. That's actually how you should start with PS_ON, but most people just have paper clips... The PSU should power on, some newer PSU will power down after a few seconds, but it's power on for at least a brief time anyway (perform a self check, etc)
    – polemon
    Sep 23 '10 at 1:55
  • OK, I've done this. I shorted the ps_on against one of the grounds and it powers up. I guess one of the components must be damaged. Any advice for checking the motherboard for basic functionality?
    – user13137
    Sep 23 '10 at 2:17
  • To clarify, I've unplugged all components (except the CPU) and there is still no activity. I also verified the power switch works using a multimeter
    – user13137
    Sep 23 '10 at 2:23
  • OK, since all you've got left on the mainboard is the CPU, you might remove that one, and check the mainboard once again. It all points to a fried mainboard, since it's quite unlikely that the CPU is broken. If the mainboard is indeed broken, you need to remove the CPU anyway.
    – polemon
    Sep 23 '10 at 8:34

Power supply is where I would go first. If the CPU was bad then at least a boot to BIOS would start. The motherboard is another canidate as well. However, what you describe is best diagnosed with Power supply failure first.

  • 1
    @KronoS: A bad CPU will prevent the BIOS from booting. Otherwise I agree with your post, though I'm betting motherboard as opposed to PSU.
    – hotei
    Sep 23 '10 at 1:13
  • It may not fully go to the BIOS but there will some sort of activity Sep 23 '10 at 1:15
  • @hotei: I'd still check the PSU, just to be sure.
    – polemon
    Sep 23 '10 at 1:40
  • @Kronos: A flakey CPU might POST. A dead one will not.
    – hotei
    Sep 23 '10 at 2:31
  • 1
    @Polemon: I'd check it too, I'm just saying the highest probability is not with the PSU. The light on the motherboard says the PSU's 5 volt system is working to some degree. Thus the PROBABILITY is that it's not PSU. I'd certainly test it but be prepared to keep going. I had a MB fail with same symptoms about 5 years ago.
    – hotei
    Sep 23 '10 at 2:36
  1. No. Think of the motherboard LED as indicating that 1% of your system is working; the important stuff is in the other 99%.
  2. No, executing the BIOS requires a CPU. However total CPU failures are extremely rare.

At this stage, it could be either the power supply or the motherboard. Most motherboards will emit beeps if any of the absolutely necessary components (RAM, CPU) is not working properly, so it's unlikely to be one of them.

If you end up swapping components for testing, it will be a lot easier to plug in an alternate power supply. If a store around you allows open box returns, it's a lot easier to make an acceptable-looking repackaging of a used power supply than of a motherboard (which might have thermal paste smudges, get bent, …).


You may want to invest in a power supply tester. I have one similar to:


Without additional components to swap in for testing, this is a cheaper alternative. It sounds like the PSU (power supply) or motherboard is dead, as someone stated previously. Normally if the RAM or CPU is dead, you will hear beeps from the motherboard. Unplug everything you can unplug (PCI cards, drives, RAM) and put things back one at a time. Note any differences along the way (beeps, fans moving, etc.). Hopefully, your monitor cable is just loose. ;)

If you do end up going the spare parts route, make sure you don't spend more than a new PC would cost (unless you're into this stuff). Often it's faster/easier/cheaper to just buy a new barebones mobo/cpu/ram as prices fall and speeds rise. Check your warranties, too, in case something can be returned.

Good luck!

  • 1
    I won't vote down, but it's a waste of money if you just want to test one unit. Grab a multimeter for 5€ or less, it's more than you need, and you can use if for a lot of other stuff afterwards.
    – polemon
    Sep 23 '10 at 1:39

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