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Here's the problem:

When I start the PC, the mainboard powers up, then stays that way maybe 0.2-0.5 seconds, and then shuts off again.

I say mainboard, and not PC, because I removed all the parts from the system and disconnected everything but the mainboard power supply (the broad 12 pin thingy).

When I have the other parts (cpu, graphics card, ram, etc.) installed and connected, the basic behaviour stays the same, but now the mainboard runs for about 6 or 7 seconds (this is a guess) before shutting off.

This all started when my monitor wouldn't receive a video signal today, without giving POSTs, so I took the graphics card and the RAM out to see if it changes anything. It didn't, except that from that point on the mainboard would start to have this behavior where it just stays on for a very short time and then shuts off again.

I already tested it with a backup PSU -> same behavior.

What could this be? I'm thinking it can't be on a physical level (transistors burned through or something like that), since then the mainboard either shouldn't start at all or it should detect hardware failures in non-essential parts of the syste and start beeping.

Sorry, I forgot to mention. It's an MSI P67A-C43. I already checked the capacitors if someone popped, but I can't find anything. I also tried resetting the cmos, but that didn't change anything.

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Have you plugged in the second power lead (usually a 2x2 plug)? See Step 3 here –  ChrisF Nov 25 '11 at 20:01
    
What mainboard do you talk about? @ChrisF, good point, I got trapped by this problem short time ago. –  Michael K Nov 25 '11 at 20:04
    
@ChrisF of course. Remember that the pc worked perfectly just a couple of hours ago. I also edited the question to mention things that I don't remember. –  heishe Nov 25 '11 at 20:11
    
In your edit you never mention you reconnected the 4-pin power connector. did you check that? –  Scott Chamberlain Nov 25 '11 at 20:21
    
Yeah, I'd go with the 2x2/2x4 (for more power hungry boards) not being properly hooked up. Additionally, I'd try start up the motherboard directly via the motherboard power button. On older buttons you have to short it to get it to start, but newer mobos have an onboard-button you can hit to turn it on (same as hitting the power button on the front). Could be as simple as a stuck power button. –  White Phoenix Nov 25 '11 at 20:29
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2 Answers 2

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It could be a hardware failure of your motherboard. Look for capacitors with bulging tops or foam leaking out of them. If the power supply detects a short circuit it will behave just like you where describing. Also there are some other self tests the CPU does before it even shows the POST screen, and it it fails those it will do what you describe too.

After double checking that you have correctly connected all of the cables (the large power connector and the 4 pin connector, also check to see if your GPU needs it's own power connector) I would try a different motherboard with the components from your PC (CPU, RAM, GPU, PSU) and if it works that you need to replace the motherboard.


There is a 2nd power connector for your motherboard, from your comments it seems that you did not connect it. JPWR2 in the below image

enter image description here

enter image description here

Does it work if you connect that cable?

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Do you have any idea why the system keeps running for a couple of seconds when I have CPU, GPU etc. installed, and just runs half a second if nothing is connected? –  heishe Nov 25 '11 at 20:13
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Well if NOTHING is connected it wont work as the CPU handles the bootup sequence, if there is no CPU there is nothing to boot. so the power supply will receive the signal "time to power up" but if nothing takes control of the boot sequence it just shuts back down. –  Scott Chamberlain Nov 25 '11 at 20:18
    
oh... that makes sense. so thats why it stays on longer with the cpu in. mhm.. –  heishe Nov 25 '11 at 21:16
    
I had that cable connected. What's weird though is that on your picture it –  heishe Nov 26 '11 at 13:05
    
I had that cable connected. What's weird though is that on your picture it's a 4-pin, while on my mainboard it's actually an 8 pin. Oh well. I think it's the main power supply for the CPU, since when I don't have this cable connected, the mainboard shuts off very quickly after I turn it on (half a second), and if I have it connected, it stays on 4-5 seconds. –  heishe Nov 26 '11 at 13:06
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It could be overheating.

I had a machine which behaved similarly (after being dropped about 6 inches unto carpet). It would sometimes last a few seconds (possibly as many as 5 or 6), but other times it would cut out a lot quicker (ie less than a second).

The problem turned out to be with the heat sink which must not have been making good contact with the processor, because after re-attaching it with some thermal transfer/glue, the problem went away.

Note, there was no visual indication of this, and the heatsink was screwed to the motherboard (not just glued to the processor) and it did not appear to be loose.

I wasn't the one who fixed it and to be honest, I thought my friend who did this was wasting his time, but thankfully he didn't take any notice of my opinion.

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I bought new thermal paste today and applied it. No change. –  heishe Nov 26 '11 at 13:07
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