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I removed the CPU heatsink/fan unit from my desktop computer and put it back.

Now when I turn on the computer, it starts to boot, gets to where it starts checking devices, then just turns off completely. I've restarted it 5 times but it keeps turning off, will never start booting windows at all.

I even completely removed the heatsink/fan thinking it might not be fastened correctly, but even without it, the computer still begins to boot then just turns off all of a sudden.

One thing I also did was remove a thick layer of (what I thought was) dust from the top of the CPU and the bottom of the heatsink. Reading the answers below, it seems that I removed the hardened thermal paste.

What can I do to get my computer to boot correctly again?

EDIT: I just turned it on again (without the heatsink/fan unit attached), it got to where it found its memory, then it said, "CPU fan error!" and turned itself off.

So I need to put the CPU heatsink/fan back I figure. But the problem is I can't get the four little pegs to fasten securely into the mother board, the little plastic tips bend this way and that and I can't get all four of them to get into their holes all at once, I've gotten three in but not all four. Is there a trick to this that I don't know?

EDIT: one of those little plastic pegs broke of, so I'm just going to get a new heatsink, thanks AlbertoPL for the tip of getting one of these so I don't have to deal with those little clips anymore.

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Solution:

I bought a new LC-Power fan for 15 euros ($20) which came with a little bag of thermal paste, was a bit difficult to get even the new pins in the mother board, you have to press so hard you think you are going to crack it, but finally worked, computer started immediately first time and fan is MUCH quieter!

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What motherboard do you have? –  stukelly Jul 15 '09 at 17:47
    
Motherboard is an ASUS P5B MX. –  Edward Tanguay Jul 15 '09 at 18:26
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5 Answers

up vote 1 down vote accepted

There can be three things wrong in your case.

  1. Stock fan not correctly fit -- there are some instructions here.
  2. You may have forgotten to connect your fan power to the motherboard.
  3. You did not cleanup the dried out thermal paste properly and the sink and processor surfaces are not connecting properly

Even without any thermal paste layer, a stock sink usually lets a processor run at almost max temperatures (you are discouraged to do this)

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so this means if I get the heat sink and fan unit to correctly fasten to the motherboard, without any thermal paste (metal heat sink touching metal cpu back) I will be able to use the computer? –  Edward Tanguay Jul 15 '09 at 18:57
    
You will be able to boot the computer. But, if you use it that way, depending on ambient temperatures (inside your chassis and outside in your room) it may overheat and shutdown. All recent processors detect overheating and trip themselves off. So, Idea is to apply thermal grease (a very thin film). –  nik Jul 16 '09 at 2:01
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Is your computer overheating? I would check to make sure the fan is operating properly. Improper cooling will cause the computer to shut itself down.

Edit: Get a mounting clip to make placing the stupid Intel pins easier.

Something like this:

http://www.newegg.com/Product/Product.aspx?Item=N82E16835117024

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yes, I think the problem is I can't get the fan to securely fasten to the mother board, or could the problem be that I cleaned off a layer of what looked like hardened dust between the contact of the fan and the back of the CPU? –  Edward Tanguay Jul 15 '09 at 17:46
    
My computer has been shutting itself off ever since my heatsink was not placed on properly, I would first try to mount the heatsink back correctly. I doubt removing dust is causing the problem. –  AlbertoPL Jul 15 '09 at 17:48
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If that hardened dust was thermal paste (it shouldn't have been), then you could try getting some Antec Silver and using a little bit on the back of the cpu. –  AlbertoPL Jul 15 '09 at 17:50
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Be sure that heatsync is properly touching the CPU as well. If you removed it, chances are you disturbed the thermal compound on it and this needs to be cleaned off and replaced.

As a note, you can get a can of compressed air to clean the fan next time. Saves a ton of time and wont damaged your CPU if used correctly.

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Heat*sink* ;) . –  Lucas Jones Jul 15 '09 at 18:10
    
Oops, guess my mind was some place else. –  Tim Meers Jul 16 '09 at 12:05
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Did you remove just the fan, or the heatsink as well?

If you removed the heatsink, chances are you removed the thermal compound and it doesn't have a good connection anymore. You'll need to clean it off and reattach the heatsink (using some new thermal compound).

It's also possible you somehow disturbed the heatsink in the process, which effectively does the same as removing it. If the fan seems to be working ok (is blowing hot air), your best bet will be to remove and reseat the heatsink like above.

I doubt it would turn off after 5 seconds even with the fan off if the heatsink were worth anything. Unless it's a sauna in your room or something, of course.

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I don't happen to have any "thermal compound" in the house, is there any common household substance that I can use until I get some tomorrow so I can use my computer until then? It just seems to grease: en.wikipedia.org/wiki/Thermal_grease –  Edward Tanguay Jul 15 '09 at 18:49
    
That Wikipedia link is the correct one. Thermal compound ensures good heat transfer between two objects and seals the gaps that pressure fitting or otherwise gluing would create. You can probably clean both sides of the contact (heatsink side and CPU side) really well and try to reseat the heatsink, making sure the contact is as good as you can get. Hopefully it'll do until you can get your hands on some paste. If it doesn't, consider leaving the case open and pointing a big fan at the CPU - it might just be that extra bit to keep it cool enough. –  lc. Jul 15 '09 at 19:29
    
one of the little plastic clips on my heatsink legs broke off, so I can't even fasten it anymore, so will just have to wait till I can get a new heatsink/fan/thermal compound. –  Edward Tanguay Jul 16 '09 at 4:46
    
Guess so unless you can fashion something temporary out of a coathanger. Those clips can break so easily sometimes... –  lc. Jul 16 '09 at 17:41
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Here is a list of things to check.

  • Check the CPU fan is connected to the correct fan header on the motherboard?
  • Did you remove any power connectors from the motherboard when you removed the fan?

Edit

The CPU fan header is at the top right side of the ASUS P5B MX motherboard.

ASUS P5B MX

You should be able to connect the CPU fan to the bottom three pins.

ASUS P5B MX CPU header

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