Hi all, I built a PC 2 years ago and it's been fine until recently. I re-imaged the HDD and now the PC will power on for a few seconds (fans start, LEDs on) then switch itself off again, and without doing anything it does the same thing again and again, then I have to switch it off at the PSU switch.

I've opened it up cleaned it from dust to make sure it's not shorting but the problem is still present. I want to avoid stripping it completely for a proper clean. However it will start normally (sometimes) with the side off, but when the case is closed it does the same thing. I can close the case once started but when I power on afterwards it does the same thing. When it does run (with case open) it seems fine, had it running overnight doing animations without problems. Other freaky things are it's switched itself on a couple of times in the middle of the night (it's connected to a UPS) , I think all this is related. Anyone got any clues, as this has me completely stumped.

The open case would bring up the issue of heat.

The auto starting might be a "wake on" feature in the BIOS.

A failing power supply can also cause weird events.

If you can get it going with the open case see where your temps are at:

SpeedFan - Access temperature sensor in your computer

Hi jephree, thanks for the response. I've managed to fix the problem (fingers crossed), the open case was a red herring and for whatever reason it seemed more likely to run without the side on.

I thought the motherboard was shorting somewhere so I cleaned it up as best I could especially around the case header pins. As that didn't seem to work I was prepared to take the whole thing apart and give it a proper clean. However I read somewhere else that it could be shorting on the USB header pins, so I removed all the devices connected to all the USB, and firewire pins gave them a quick clean and it seems to have fixed the problem.

I've now re-attached all the USB devices and it's running as it was so all being well that will be it and if anyone else has similar problems I hope this helps.

Thanks for the update and tips.

Let us know if we can help further.

Well after a few days of me putting the PC back together, and finally putting it back in it's hole (my desk) it was working as it should but today it's started doing the same thing again. I was in the middle of something when I heard the distinct noise if the HDD, not the usual clicking but like a 'burp' then a message saing I should save before shutting down.

So saved my work and powered down. When I hit the power button it starts for about 4 secs then stops, and it tries to power itself up without me touching it. When I switch it off at the mains then switch the mains back on, it will power itself on again without me touching it.

I've disconectted all the USB headers again and the first time I fired it up it started itself and swithced off. A quick blow of the mobo and it's back to normal so I think I'm on the right track with the system shorting somewhere. I may have to take it apart and give it a proper clean after all.

Any suggestions would be great, especially if it saved me from taking it apart. If not I'll post my findings after the overhaul

The auto start might be a BIOS setting to auto start after power loss. If this is set then it would try to turn itself on after any indication of a power loss.

The 3 or 4 second issue reflects the fact that holding the power on button for 3 or 4 seconds is designed to shut off the computer. If the BIOS is reading this as a power failure then the BIOS would try to turn it back on.

When you say you put it into it's hole it again raises heat issues.

Overall there is a growing suspicion in my mind of the PSU.

You can get a PSU tester at most any computer store such as:

LCD Power Supply Tester : PSU Tester : Maplin

Originally Posted by jephree

The auto start might be a BIOS setting to auto start after power loss. If this is set then it would try to turn itself on after any indication of a power loss.

The 3 or 4 second issue reflects the fact that holding the power on button for 3 or 4 seconds is designed to shut off the computer. If the BIOS is reading this as a power failure then the BIOS would try to turn it back on.

When you say you put it into it's hole it again raises heat issues.

Overall there is a growing suspicion in my mind of the PSU.

You can get a PSU tester at most any computer store such as:

LCD Power Supply Tester : PSU Tester : Maplin

Thanks again jephree, I'll look into the BIOS. I don't quite understand your explanation about the 3 or 4 second thing. Can you explain again please. I'll try and expain what happens so maybe you can relate it to that. The PC is powered by a UPS which I use as a master switch. So when I want to use the PC I switch on the UPS, sometimes the PC will power itself on without me touching the power button and sometimes I will need to press the power button to switch the PC on. Eitherway the PC powers up the fans will spin then after 4 secs everthing stops (LEDs fans etc, like power has been cut) then without touching anything the PC will power up again where the same thing happens again. Sometimes after 3 restart reboots, on the third attempt it loads into windows (after which it will turn itself off, power failure off), other times it stops and doesn't do a thing, just dead.

When I say put it back in the hole I just mean back into the desk, it's an open desk no door or anything like that. Recently when I've had to do all this stuff I need to pull the case forward onto the floor, whereas normally it just sits in a space in the desk about 2 inches off the floor, a very standard desk.

The PSU tester will be cheaper than getting a new PSU so if I'm still going roudn in circles I may have to resort to that.

Just as a test I would temporarily remove the UPS from the equation.

Any computer will shut off if you press and hold the power button for 3 or 4 seconds. My theory was the power button was cutting the power and the BIOS was set to reboot from power failure.

Anyway see if removing the UPS has any affect on the behavior.

I have tried removing the UPS from the equation, and no change, however I think you're right with the power button. I've disconnected the power botton switch from the board and start it up by shorting the pins with a screwdriver and so far it seems to be behaving as normal. So I think that could be it, I've since swapped the power SW and reset SW to power it up that way and so far so good.

All being well I will post a final report after a week and we can put this to bed.

I've replaced the the power switch cable and that seems to have fixed the problem. It's been running fine for over a week although I have still to reconnect the USB devices to teh header pins but I don't think that will cause a problem.

Many thanks jephree, didn't know that about holding the power button but that described what I was experiencing.