Dead System -- PSU or MB?

[Sal Ammoniac]

I've got a system built with an Asus X79-Deluxe motherboard and a Seasonic Platinum PSU. It has an Intel Core i7, 64 GB of DDR3 RAM, and a Samsung SSD as the C: drive.

I left the system running overnight last Saturday and when I went to use it Sunday afternoon it appeared to be powered off. The power switch on the computer's case had no effect--the fans didn't start spinning and there was no output on either of the two monitors. I verified that the wall outlet it was plugged into had power (it did).

I disconnected all external peripherals (monitors, keyboard, mouse, Ethernet, and speaker) and tried again. Still nothing.

I opened the case up and noticed that the power button on the motherboard itself was lit (as was the reset button next to it). This indicated that the system was getting some power from the PSU. I tried pressing the power switch on the motherboard and that didn't have any effect. I next tried resetting the CMOS (by holding down the "reset CMOS" button on the MB while pressing the power switch, but that had no effect either.

Is this issue likely to be a power supply issue or a motherboard issue? Unfortunately, I don't have a spare PSU lying around that I can try. I'm hoping that it's the PSU, though, as that would be much easier to replace than the motherboard.

[Psi]

Try unplug the PSU from the motherboard and then force the PSU to 'on' state by shorting the PS_ON pin to GND.  (the PS_ON wire is usually green).

If it wont turn on (spin up fans) then the PSU is dead.
If it turns on then then the issue is your MB not telling the PSU to turn on. This could be a dead MB but could also be that one of the many voltages from the PSU is missing or bad.

Also check the PSU 5V VSB rail and make sure it's 5V under load.
This rail should be present even when the psu is off and if it dies the motherboard wont have the power to turn the PSU on.

[wraper]

Short green wire to GND (24 pin connector) to switch PSU ON without any motherboard attached. Then measure voltages or at least look if fan starts spinning.

[magic]

Also check if the POWER_GOOD signal is coming from the PSU. I had one which failed that way even though all voltage rails were fine.

[Psi]

Also check if the POWER_GOOD signal is coming from the PSU. I had one which failed that way even though all voltage rails were fine.

I had one fail where the +5 VSB would read 5V except if you try to draw more than 100mA from it.
It was annoying because those little ATX psu testers you can get fail to detect this fault.
So when i returned it they used that to test it and then said it was fine.