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I've been having a problem with my Ubuntu 12.04 64-bit machine which is happening quite often and randomly. During a normal session and without any indication that something is wrong the computer just shuts down completely as if I unplugged it from the wall..

I read somewhere that this is probably caused due to overheating so I installed the "sensors" program to monitor the CPU temperature which running it after the crash indicates that all Core temperatures are at normal levels.

Does anyone have an idea as to what might be causing the problem and most importantly if there are any log files I could examine to troubleshoot this.

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Take a look at the contents of /var/log/syslog just before the messages pertaining to your most recent boot/startup. There is also a Log File Viewer GUI application. You can locate and start it by searching for log file in Dash. Have you considered that it may be a hardware problem? Perhaps in your Power Supply Unit (PSU)? –  irrational John Jun 15 '12 at 4:06
    
Can you please give as many details as possible about your computer? It is very difficult to solve your question with so little information. Thank you. –  Brask Jan 9 at 16:37
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3 Answers

This is what I would do in your situation

The Problem: "It randomly shuts off while im doing something..." *as if i just unplugged it.

First(if possible) get to XTerm, mess with preferences and create an infinite scrollable history. (click edit -> profiles -> highligh default -> edit -> scrolling -> check unlimited scroll. (if you already know how to do this, so sorry for the redundancy) Then type "dmesg" enter (maybe sudo su first) What I would do, Is scroll through it until i see an event I recognize, like "Starcraft 2 running" in my case, or something.

****Beyond this line on is bonus extras you dont have to read.******

READ ALL QUESTIONS BEFORE TRYING ANYTHING These are troubleshooting steps

Question: Is it a power fluctuation? How to answer: Change outlets to see if it still happens, perhaps theres a short that has occurred recently.

Question: Could it be power management? To solve this, go into your bios and disable all power management functions (and overclocking if applicable) this is stuff like "tristate" "ddr power down" "c6" "apm" etc...

Also look for any "shutdown on temperature or (any event) like fan speed running at Xrpm.

If you add more detailed information, I might be able to help better. This answer might not be what you wanted, but you have a very non-specific issue. and i understand. But you have to eliminate possibilities.

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When you say "normal temperature" what temperature is it? Different processors have different temperatures at which they shutdown.

Sometimes old computers have "earthing problems". Some of their circuits get accidentally grounded. When you touch the computer at some point (such as aluminium designs on the surface) the circuits short and the computer just shuts down.

This is a problem on my old acer extensa which immediately freezes and shuts down when i touch it aluminium strip near the screen.

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If it does a clean shutdown, and it is a temperature problem as you guessed, that is usually indicated in /var/log/syslog as mentioned by irrational john above. I had this with a laptop and a failing CPU fan. So if e.g. it says there it's shutting down because the CPU is overheating, check also whether your CPU fan is working. Same for the other fans if anything is overheating.

On the other hand: if it simply powers-off without a clean shutdown, it rather points to a hardware failure. I had this when one core of my CPU gave up on another machine. In those cases, there won't be a related entry in the syslog (as there was no clean shutdown, but just a power-off). Here you could run some hardware checks:

  • boot to memtest to run a complete RAM check
  • use S.M.A.R.T. capabilities of your harddrive for a health check (e.g. with the smartmontools package)

Just to give a few examples.

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