• Resetting the iMac (Flat Panel G4) Power Management Unit (PMU) - Apple Support
The later G5 PPC iMac model, pre-dating early Intel-iMac, uses a different idem
because their chip is not called a PMU. Some of the links to various power chip
reset articles may no longer work from this page; the chip names are still correct.
•Resetting the power-management chip in your Mac:
http://www.thexlab.com/faqs/resetpmchip.html
There is a tiny reset button next to the test ports under the metal disc bottom plate
cover in the iMac G4 series. Usually the service support documentation covers this.
{I have an original Apple Service Manual in PDF I used to self-diagnose my iMac G4
under warranty because the only shop that would see it under AppleCare refused to
& so I had to do most of the troubleshooting myself. And Apple told them what to do.}
(The reflow of solder for graphic chip in G5 iMac + some intel iMac was more common
as was attempts to do the same with some white iBook G3/G4 models, than iMac G4.)
Oh: The battery is in the bottom on the logic board, the power supplies are in the top.
Heating from the top may be slow to help the logic board directly if solder is bad.
These models usually require a known-good PRAM (clock) Battery installed; so
if the unit has sat around a few months unplugged, by now it probably is dead.
This battery does not re-charge while in use: a 1/2AA 3.6V Lithium. Cost, to $15.
• example: 1/2AA 3.6V Lithium: for some models, incl. iMac G4: $3.95:
http://eshop.macsales.com/item/NewerTech/BAA36VPRAM/
The PRAM and NVRAM reset sequence uses the same keys; recent article, here:
• How to Reset NVRAM on your Mac - Apple Support
...Anyway, should someone reference this site, in this year, above links may still
be of some help. Resurrected ancient threads with dead links are of little help...
Yet some vintage ones - that should be archived - may re-appear & serve to confuse. 😝