My ASUS A8N-SLI Premium has developed an odd problem. If the power supply, Antec SmartPower 2.0 500W, has been unplugged or switched off the case switch will no longer start the machine when the power is restored. If I ground pin 14 (PW_ON) on the ATX connector everything spins up normally. After that the case switch works normally for turning the machine on and off. If I unplug the box I need to open it up and manually ground pin 14 again. It's a huge pain in the ...
Is this a motherboard problem or a power supply problem? I'm planning to rebuild the box and was going to reuse the power supply. Now I'm not sure.
Beyond what r_manic suggested, which is a good suggestion by the way, the problem is most likely the PSU. You didn't state the age of your present PSU, but the clue I see is that after grounding the 14 pin from the PSU, everything works fine. This indicates to me that the motherboard itself is fine. You can get a good PSU fairly cheap these days, and most likely one that has better efficiency so that your electrical power bills will drop slightly while at the same time having the heat produced inside the computer go down.
If by chance the computer powers up fine from a different wall socket, you have a potentially more serious problem. That would indicate faulty house wiring and it needs the attention of an electrician before something worse happens.