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I am trying to fix my old desktop that stopped working this past summer. When it stopped working, I was playing a video game and the computer simply shut off.

Here is what happens now when I try to boot my PC from a powered off state by pressing the power button:

  • My CPU fan twitches slightly, but doesn't spin
  • All other case fans spin up as normal
  • All of the phase LEDs on my motherboard light up and stay on
  • My HDDs and disk drives sound like they spin up normally

After this, nothing really happens. My monitor doesn't receive any display, even though it's on, working on my other PC, and I've tried both of my video card's outputs.

The oddest thing is when I go to shut off my PC. When I pull the plug it simply goes dead, but when I hold down the case power button for 3 seconds, this happens first:

  • My CPU fan spins up immediately then loses power again and slows to a stop after a second

My first thought was that my CPU fan was dead, but it seems to be working, just not on. I tried connecting it directly to a system fan connection and it spins up and continues spinning without problems.

I have reset the CMOS for my motherboard, and my next step is getting a can of air and resetting all the components in the PC. What could cause this odd CPU fan behavior?


Motherboard - Processor (w/ stock fan) - GPU - PSU

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It could be that the fan bearing is damaged. Most computer have fan speed sensors that prevent the computer running if the CPU fan has stopped. The fact that the fan twitches but stays still, suggests it is getting power. When the computer is restarted in certain ways, (such as the power button trick above) the fan briefly runs at full speed/power (which may be enough to overcome the friction in the bearing). Try to swap that fan for a different one of a similar type, just momentarily and see if you can power up. –  washbow Mar 6 '13 at 2:23
    
Is the fan connected by a 2 lead cable or 3? Does it go through any kind of adaptor to get to the motherboard? –  washbow Mar 6 '13 at 2:38
    
It's connected to the CPU_FAN connection via a 4-wire cable. When it was connected to the SYS_FAN connection there were only three pins. It connects straight to the motherboard. –  dlras2 Mar 6 '13 at 2:43
    
It could still be the case that the fan bearing is damaged and won't spin at low RPM. The 3-pin SYS_FAN header would run it at full power, whilst the pulse-modulated CPU_FAN header is speed controlled. Is the fan stiff to turn with your fingers? If possible I would try connecting a different fan to test. –  washbow Mar 6 '13 at 2:46
    
I'm fairly certain now it's not the fan; it's never been stiff to turn with my fingers, and is not so now. I may still try connecting a different fan just to be sure, if I can get my hands on one, but this isn't my current priority. –  dlras2 Mar 6 '13 at 3:00

3 Answers 3

There are only two reasons I can think of:

  • Unlike the case fans, the CPU fan is completely temperature controlled. I suspect the CPU doesn't generate any heat (i.e., it's dead or it doesn't receive power), so it never reaches the minimum temperature to turn its fan on.

  • The fan is actually damaged and the computer refuses to turn on to protect the CPU.

Connecting the CPU fan directly to a 12 V outlet (e.g., a spare case fan connector on another motherboard) should let you rule out a damaged fan.

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I've confirmed the fan is working; see edit. –  dlras2 Mar 6 '13 at 2:34
    
Then my money is on dead CPU. Do you have another system with the same socket? Exchanging processors should rule out other less likely scenarios. –  Dennis Mar 6 '13 at 2:48
    
I can try to find another CPU like mine. However, shouldn't I get some sort of beeps from a dead CPU? –  dlras2 Mar 6 '13 at 3:06
    
If you have a PC speaker, then yes. But pretty much any other problem that prevents the computer from booting should generate a beep code. –  Dennis Mar 6 '13 at 3:29

On some much older PCs I have seen CPU fans that only spin when the heatsink reached a certain temperature threshold. There was an option in the BIOS to override this and force it to spin constantly. In my case disabling "Cool n quiet" in the BIOS lead to the fan spinning constantly- rather only at certain times.

Check the BIOS and see if you can find any options which might relate to the CPU fan being disabled at lower temperatures- and see if overriding this makes any difference.

Also, try connecting another fan to that fan header and see if it spins. I would be interested in ruling out fan failure in order to get a better idea of what was going on.

Once you have confirmed the fan is working- then you can start troubleshooting the PC as a whole. With a bad CPU fan, it's entirely possible the PC will shut down power to the CPU to protect itself. There aren't many occasions besides thermal issues or detected fan problems that will lead to a CPU being automatically shut down- so a bad fan is the prime suspect here.

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I can't check the BIOS because it won't ever get there. And the PC's not what I'd call "much" older - no more than 3-4 years. –  dlras2 Mar 6 '13 at 2:29
    
When the fan is "twitching" at startup, see if giving it a spin with your finger helps it to overcome any bearing friction. As another commenter posted- perhaps it tries to spin faster at shutdown, which could be overcoming friction. If you can rule this out things will be easier. –  Austin ''Danger'' Powers Mar 6 '13 at 2:38
    
I tried giving it a spin with my finger already. And as I said, it spins just fine when connected to a case fan connection. –  dlras2 Mar 6 '13 at 2:41
    
At least we know your fan is perfectly fine. Does your monitor even come out of power saving mode? Or is there no graphics output whatsoever? Also, try disconnecting all drives from the motherboard (hard drive, DVD drive) as well as all peripherals from USB ports. If it still can't even POST, try disconnecting the AC power, removing the CMOS battery, holding the power button for 1 minute, then putting the battery back in and powering up. If it still doesn't work you could be looking at a motherboard failure. –  Austin ''Danger'' Powers Mar 6 '13 at 2:46
    
My monitor stays in power-saving mode the entire time. I will try removing all the drives. I've already cleared the CMOS (I think - I followed the instructions for shorting out the jumper pins.) –  dlras2 Mar 6 '13 at 2:52

Your PC. The system is starting up, something is going wrong and (the motherboard) is shutting down - leaving the drives etc spinning. The problem has nothing to do with your fan.

One thing which occasionally helps is removing all the non essential bits in the system, and if you are lucky it will power up. Then turn off system add bits incrementally until you find what is not working (although in all likelyhood its not going to work). If you do try this, ensure you remove all the memory in excess of what you need. Also, if your system has more then 1 stick - and can support 1 stick, try alternating the sticks - I've had systems die because of memory failure.

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The motherboard appears to remain on until I power it down, though. –  dlras2 Mar 6 '13 at 3:27
    
If the motherboard was shutting down, the whole PC would be shutting down. The motherboard isn't shutting down. –  Austin ''Danger'' Powers Mar 6 '13 at 3:52

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