Give me an EMC #. It's located at the bottom of the door. What is your current RAM configuration (this is critical) how much and in what slots? Have you changed the battery? Do you have a volt meter? We may be testing the power supply output later without tearing into anything. You don't want to replace the power supply because you don't have the CPU synchronizing software that is on the Apple Service Disk (ASD) that only authorized service providers have. I've been trying unsuccessfully to get one for two years. Without this the fans run wild when the power supply is replaced. I own this machine and just put a new video card in it today. So I'm familiar with it.

Here is a URL for a take down guide for your computer. Hope it helps. Ralph

http://g5.bhmm.net/

Try removing the graphics card, and putting it back in. If it is installed incorrectly, or just knocked out of place this might solve the problem.

On my old G5 2Ghz, I had the same problem.

The solution (for me) -> plug a VGA monitor with DVI adapter to the graphic card, and it boot when I plug the cable...

I know this is not a thing to do (to plug or de-plug to/from a graphic card when the mac is on, but that was the only thing to power on/down my mac)

Without a monitor plug (or if a monitor is already plugged) , no way to start the G5

It's like there is no power, start switch does not go on.

To shut it down, I needed to de-plug it after the "Shut Down clic" in the finder (elsewhere the G5 reboot but no shutdown)

That was a really, really strange bug...

Until the day I changeg my graphic card for a 6800 Ultra DLL and since that day the power button works perfectly. (same thing for the shut down, no need to de-plug video cable during the shut down process).

Update

Few more informations, this G5 DP 2 ghz was a PowerMac with the "shut down" symptom.

It works for few months without other problem and one day it shut down randomly.

It can power on few minutes/hours/days and suddenly shut down without any explanation.

I opened it, change the 3.6V, reset PMU, ... and nothing change.

According to the Apple center the problem came from the PSU.

A month later they said that was the mother board...

I opened it again and clean the dust.

Same thing.

Opened it again, and I pulled and press different components on the mother board, and the G5 works very well for months.

One year later I added a second Hard Drive, RAM and a PCI-X board and some days later the problem reappeared... the mac shut down randomly again !!!

One more time in the case to clean up and to pull and press components to make sure there was not a unsoldered piece.

Since that day, this "randomly shut-down problem" disappeared, and that was more 3/4 years ago.

This G5 DP 2 Ghz turns every days, almost 24/24 without problem.

i never changed the PSU, never changed the MotherBoard and nobody in Apple center had never explained why was happen and what the problem was.

And I'm not alone, a lot of people with G5 had this mysterious bug.

Apple always denied the problem and the only words from us was "change PSU, motherboard" said to me an Apple repairer, because this shut-down bug was never clearly identified on the G5...

Sorry for my long post and my poor english ;)

There's a whole host of manuals available for macs, at http://www.eserviceinfo.com/ but for the G5, try this link at http://www.eserviceinfo.com/download.php....

It's a multi-part zip file, but it might help. Very detailed, with pictures. I've currently got a PowerMac G5 1.8 Dual-Processor with 4 GB RAM Maxed out, and I'm having problems with this booting up. I've got a bit of free time over the next week and over xmas so I'll give it a go and strip this down and figure out what's what.

If you've tried the graphics card, memory and any other non-stock peripherals, my guess would be the logic board (as its not receiving power properly enough to boot) or its your power supply. I'd put money on it its your board or power supply.

Had a similar problem with the 1.8 dual processor G5 that I'd just re-built from two basket cases. It powered up the fans but with no chime and no further action. Upon advice I checked the ram modules and got two matching and plugged one into the upper of the two lower slots and one into the lower of the upper two. Turned it on and bingo, fully operational!!

Thanks so much for the above post/s, was having the power light come on my G5 1.8Ghz with 2G of ram but that's as far as it went with no screen appearing at all. Ordered & installed replacement battery, which made no difference. Like the post by P.B above I have just removed some of the RAM (but i left matching 512mb's in the top slot of the top 2 and the bottom of the bottom 2) It has fired up first time and I'm now backing up irreplaceable files i thought may have been lost. The fan is pretty noisy now but even if it only works for the next 2 hours i'm grateful,thanks again !