PowerMac shuts off during bootup

Question

This morning I woke up to find that my G5 tower was powering off during boot ups. It would turn on the, I'd hear the apple chime, the fans would start, nothing out of the ordinary, then the gray screen with the apple logo would pop on screen as per usual, but then a second or two later I'd hear the "click" that comes just before I'd turn off my computer coming on when I didn't turn it off.
I've turned on the computer 2 more times and the same result. It turns off at the exact same place everytime. No sooner, no later.
My Power Mac G5 is the last model made just before the Intel upgrade. I made no software or hardware changes recently that preceded this issue. I pressed the SMU button to no effect.
Suggestions?

Answer 1

Been there, done that (see thread link at the bottom). Here's my initial suggestion mix for this problem.
Have you tried a Safe Boot (hold down the shift key while booting)? If it comes up in Safe mode, then I would suspect a power-supply issue, but that requires a little more troubleshooting.
If you do get it up in Safe mode, look at your system log file using Console. (Click the Logs button which will reveal more log files.) Look for a log entry along the lines of (you have a later machine and are running Leopard, so the nomenclature might be different):
*localhost kernel[0]: ApplePMU::PMU forced shutdown, cause = -122*
Now, where to go from there assuming you start up Safe OK. What is probably happening is the machine is probably executing a test in the normal startup that is "flexing" as it were the power supply, which is not responding quite like the SMU expects. This may be caused by something as simple as dust accumulation in the power supply. The G5 supplies can build up a considerable amount of dust, which needs to be blown out. This might be able to be done with canned air, realizing that you will stir up a bunch of dust, so it should be done outside somewhere.
Also, you can get more info by doing the verbose boot, holding down the command-V combination when powering up. You will get a dumb terminal display of what is going on. This will verify at least that it is truly powering down at the same place in the boot. There are three ways to set verbose boot as a default for the machine, should you decide to do this.
Method one requires using terminal (you can do it after safe boot) and using the following unix command:
*sudo nvram boot-args="-v"*
You can also use open firmware, in the following manner
*set-env boot-args -v*
Or you can boot single-user mode (command-S at boot, then issue the terminal command above without the sudo, which is unnecessary in single-user mode.
I just went through a similar situation a month ago, see http://discussions.apple.com/thread.jspa?threadID=2452255&start=0&tstart=0 and I ended up replacing my power supply, though the dust blowout might have worked as well.