My daughter loves Minecraft. She came to me last week complaining that the computer she plays on kept turning off. Time to fix it.
I knew right away what the problem was from the death rattle in the case. The CPU fan was dying. Easy fix. I went to Best Buy and bought Antec 80mm fan. Installed it and Viola. Fixed?
The box still kept shutting down. Odd. I have been meaning to install another video card in it, so I swapped in a new (old) video card and a new (old) 500 watt power supply. Fixed?
No, still not fixed.
Time to do this the proper way, actually diagnosing the problem instead of just throwing parts at it.
I downloaded and installed HWInfo from http://www.hwinfo.com/. This app told me the power supply rails were fine, but that the CPU was running really hot. Really HOT! it was at 70C just running Windows. Bad times. Before I tried anything else I wanted to be sure that overheating was the problem. I installed the demo of BurnInTest from http://www.passmark.com/products/bit.htm. Running the CPU test the temperature quickly climbed 78..79... 80... and powered off. So that's definitely the problem. The PC is shutting down to keep from cooking the CPU.
I had a shiny new blue-LED for-extra-coolness Antec Fan on it, why is it overheating?
... Because I neglected one important step. When I pulled the heatsink to replace the fan I neglected to replace the heatsink grease. Crap.
I headed back to Best Buy for some thermal grease. Back home, I cleaned off the old crud on both the chip and heatsink with Q-tips and Rubbing Alcohol. I then put on a thin layer of new thermal grease and screwed the heatsink back on.
Firing the machine back up, It stays at 55C when running a CPU burn-in test now with the fan control set on the lowest (quietest) speed. Fantastic. I did a quick experiment to see if pulling air through the heatsink or pushing air through the heatsink would result in cooler temperatures. Pushing air through the heatsink was the winner by 2 degrees.
Lesson Learned: If you pull a CPU heatsink, replace the thermal grease. It can easily make a 25C+ difference.