How do I test to make sure my fans are working properly

You can get a program called XResourceGraph (free) at

http://www.gauchosoft.com/Software/X%20Resource%20Graph/General%20Info/

It shows temperatures at various locations inside your Mac. I installed it on our G5 iMac. It also shows how fast the fans are spinning (rpm). You can watch the temperature start to rise for the hard drive, or the processor etc. and also simultaneously, you will see the appropriate fan increase in rpm and then the temperature stabilizes or comes back down. Mine stays between 130 and 150 degrees F for the processor. If one of your fans shows as 0 rpm, you will know it is broken. My fans run between 1500 and 2000 rpm mostly, but when doing a lot of stuff, I have seen them up to 4000 rpm, while the temperature stays constant (presumably because of the fans).

I think these fans and the need to actively cool the iMac G5 is a weakness in the design, although it's a great computer. In contrast, the much cooler Intel chip in the new iMacs has very little need for such active cooling. I can't hear ANY fan hum at all in my daughter's new iMac.

This would be important to get fixed if it is the case because the G5 PPC chip throws off a lot of heat and your computer is likely to overheat sooner rather than later if one or the fans has stopped.

If the fans were not working properly the iMac would overheat and shut itself down.

-DP