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I have a desktop PC that is now refusing to switch on properly. The fan will come on, but then about 3 to 5 seconds later it shuts down. Pressing the power on button a second time does nothing, not even getting few seconds of the fan running. But if I pull out the power cable and plug it in again, then the power on button does work again - though again only for a few seconds.

Any ideas?

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As other have said, lower the load. Try with only motherboard and HDD connected. Also, check the inside of your computer for any places where exposed wires or solder joints may touch the case. To me it looks like the problem may be short-circuit protection kicking in after few seconds.

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This is often indicative of under specified PSU (or PSU failure).

Have you recently added new hardware? How old is the PSU?

Try (temporarily) reducing the start up load (e.g. reduce to a single graphics card (or lower specification card), hard disks, ...) and see if that helps.

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Its a four or five year old PC. I added new memory about 6 months ago but it has been working fine ever since. – Mick Sep 26 '10 at 12:46

Sounds perhaps not quite but a little bit like what would happen if forgetting to plug the 2x2 cable into the motherboard. I think I had it with the P4.. an old system.. probably your system is more modern and perhaps only the motherboards for the p4, that needed it, but anyhow, if your motherboard has that socket, then plug it in.. I think in the p4 case perhaps it always did the few seconds thing. didn't need to unplug it and plug it back in for it. so perhaps not quite the same symptoms.

Also take the motherboard out of the case and try And remove screws from it.. (I did once have a weird MBRD that didn't power up at all 'cos one screw in a hole designed for a screw, but not meant to take it.. shorted it. was strange. I don't think your case.. but worth trying to take the MBRD out of the case)

And do try another PSU. that's an obvious thing to try.

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