iMac G5 shut down on own!

[thertner]

We have had this desktop for 4 years and had zero problems with it. I sleep the unit whenever i leave it for an hour or more. But yesterday it shut down completely in sleep process, after returning to it i noticed this. One time upon boot the fan took off like airplane, never heard that before. TechtoolPro4 says "i have 3 bad blocks", and to wipe data and reinstall. Does this seem correct or should i run some other type of diagnostic? Any thoughts much appreciated!
Thank you.

 

[Ambrose Chapel]

this happened to my iMac G5 and it was due to the power supply failing. if the spontaneous shutdowns happen repeatedly that may be your problem. not sure about the techtool diagnosis though. that may indicate that it's a hard drive issue

 

[Dave00]

There are two possibilities; power supply failure or logic board problem. This machine had big problems with the logic board, they replaced mine twice for free because it was a defect with the system. Later, I needed a new power supply, which was something like $150 at the apple store (for installation, part, etc.)

Dave