iMac G5 PowerPC won't start with power button

Hello Sean-John-Michael-and-Miriam-- I've continued to experiment since my last post, and (by following the advice of kind Miriam)... as my computer has continued to have bouts of not responding to the power-button at all, (intermittently), I've started the following process that then allows my computer to boot every time I encounter this problem:
1. I start by unplugging all the peripherals (in my case that's a Lacie superdrive plugged into a firewire port, a 4-port USB hub, [with just the plug-adaptors attached for my ipod, & cameras], and a Canon Pixma IP4300 printer).
2. Then I unplug the main power cord from my IMac G5, and re-plug it back in.
3. Then I hit the power-button and it starts every time!-- After it's started I plug back in all my peripherals... Go figure!?? Guess there's some type of electrical-glitch, (or software incompatibility?) between one of the peripherals and the "start up" functions of the Imac? Seems strange to me, but I'm glad it works consistently! Good luck John solving your non-start-up mystery-- hope it's as simple as going through this process.

You simply loosen the three captive screws at the bottom of the case, and remove the back. Apple provides directions to do that here.

If you send an electronic mail message to the address in my Public Profile, I can reply with a note which will show you where to look for evidence that you are affected by the issue.

Welcome to Discussions - try starting while holding down the Option key. You can also remove the power cord and all peripherals, wait a minute or two, plug the cord back in and then try starting again.

Is it starting at all?

Your power button is simply a mechanical extension of a circuit in your power supply, and could not—itself—have been affected by a power sag or surge. It is far more likely that your power supply has been affected by such an electrical condition, though it is also possible that your particular machine is affected by a 'defective electrolytic capacitor' issue.

Though that failure would also account for the historic failure to start consistently, it would not be directly related to the power failure that you recently experienced. A visual inspection of the clusters of electrolytic capacitors on your logic board for evidence of bloating or leakage will determine if you are affected by the issue.

I've been having a similar issue with our iMac G5 and we were having the same problem just now and starting it by holding down the option key and pressing the power button worked fine. However we had issues with it before, not waking up from sleep or shutting down unexpectedly while trying to wake it up. Could that be something related to this capacitor issue?

It's possible, though most situations involving defective capacitors are limited generally to the first generation of iMac G5 17 and 20 inch models.

As I indicated above, if you send a message to the electronic mail address in my Public Profile, I will reply with a set of instructions to help you determine if you are affected by the issue. Please include the serial number of your machine in your request, in order to properly identify your machine and aid in more clearly identifying how you might be affected.

After racking my brain and doing much online research, I finally stumbled upon Tomasso's advice.

I have the same situation where my computer will not turn on after being in "sleep"mode, instead it shuts down. I have now had to shutdown after each use and subsequently turn it back on with the power button each time. I did the diagnostic test and found that it was not the power supply.

The power button has become very sporadic and I have had to be patient as the computer eventually powers up with certain patterns of clicks or holding it in. Lately though, even these strange combinations have not been working. I did note that when I took the cover off (and unplugged all the peripherals) I could restart it EVERY time using the internal power button on the board, but as soon as I reinstalled the cover and plugged everything back in, it would be back to where I started---very sporadic powering up. That is what had me baffled.

Following Tomasso's advice I unplugged every peripheral and one by one figured out that it was my iSight camera (plugged into the FireWire port) that was causing the unique and annoying issue. After unplugging the camera, the iMac has started every time.
Hope this helps someone else that may be struggling with this problem.

I have had a similar problem once, and i found that unplugging my iMac, holding down the power button for a few seconds(while it is unplugged), re-plugging it in, and then pressing the power button again turned it on. It may sound crazy, but it worked for me. It might not work for you though.