[Reenee]

This problem has become incredibly annoying.
Basically, I put my iMac to sleep. And then suddenly, my iMac "wakes up" on its own after a random amount of time (anywhere from 20 to 90 minutes) except that the screen is not on and its fans are suddenly going out of control at their maximum speed. I have even erased my hard drive and started anew and applied all necessary updates and it is still happening.
This out-of-control fan blast happened RARELY, like after a system update or an out-of-nowhere kernel panic. But now it's so frequent, I have to shut down my iMac entirely.
I also cannot wake up my iMac with my keyboard anymore. You can hear the hard drive spin up but the screen is not turned on. I have to press the iMac's power button to wake it up.

[Reenee]

I just want to add that zapping the PRAM did not work for me.

[macnuke]

DVD that came with your iMac....
boot into the Apple Hardware Test.
run the extended.
may be bad RAM in one of the higher registers.
may be something else in the process of failing.
hope you have Applecare on it.
mine had a mobo and the powersupply replaced in one fell swoop at about 13 months.
no probs since then.

[Reenee]

No, nothing wrong detected.

[nnager]

Link to Apple knowledge base article on how to reset SMU
I don't know whether the SMU reset applies to iMac G5's but it sure fixed the jet engine fan noises on my Dual G5 2.0. You just have to remember to remove the power cord from the back of the Mac, not just from household electricity and to leave it unplugged for the specified time.
There also have been reports of changing System Preferences/Energy Saver/Power Adapter settings/Processor Performance. In my case, I've got the setting at "Highest." Others reported success with other settings.
Respectfully, Norm

[Reenee]

I think you meant to link to the iMac SMU.
I should attempt pressing the button on the logic board itself. I used my computer after doing what I believe had reset the SMU. When I was done, I did note that I did put my iMac to sleep to see if something happened.
An hour later, to check up on it, the iMac had actually shut itself off. It managed to go from a Sleep state to complete shut down.
And also, attempting to wake it from sleep resulted in the same "unresponsive, screen not turned on" state.

[Reenee]

I must unfortunately say that a new problem has arisen.
I reset the SMU as stated here: http://docs.info.app...l?artnum=300908
Now any time I try to put the Mac to sleep by way of the Apple Menu, the Power Button, and even the keyboard (Command-Option-Eject), it immediately shuts down. Not a normal shutdown either. It's as if I held down the Power Button for a few seconds to force a shutdown and you know how something can go wrong doing that if it's not necessary.