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I have a PC that died tonight after I swapped a power supply. Putting the old power supply back did not help.

Symptoms: Fans and lights come on normally. There are no beeps, but I don't see any speakers on the motherboard and I don't remember if it beeped in the past. The monitor stays in no-signal low power mode.

Significantly, the front panel power switch will turn the PC off. I believe this switch requires CPU intervention to work properly, so that indicates the motherboard and CPU are working properly - or am I missing something?

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What it sounds like you are describing is a PC that is not passing its POST. You should get an internal speaker or a PCI POST card: amazon.com/gp/product/B001O76WLY/… –  MaQleod Apr 15 '11 at 5:55
    
@MaQleod, thanks for the tip. I bought a POST card for a lot more than it was worth, and the results didn't tell me more than I knew before. It was still a good suggestion though. –  Mark Ransom Apr 16 '11 at 3:41
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up vote 2 down vote accepted

On most systems, the BIOS will shut down the PC if the power button is pressed before control is passed to the operating system. Otherwise, a signal is sent to the OS to shut down the computer.

In either case, the CPU must be functioning to respond to this button, since the BIOS is itself code that is run by the CPU (see Wikipedia). The only time the CPU is not involved is when the power button is held for four seconds, which shuts down the computer immediately by means of the motherboard electronics.

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Ah, I didn't know about the 4 second rule. I think that explains why it's still working. –  Mark Ransom Apr 15 '11 at 13:13
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