Power problem was browsing the net then pc hang. reboot it, cpu fan turns son, power supply turns on, front led lights, then power trips or pc turns off

sounds like eletrical problem try with a other PSU.

OverheatingWhen the CPU overheats then it will shut down (i.e. thermaloverload) and won't boot up again until it has cooled down. If this is theproblem is not fixed then the CPU will eventually fail.
A faulty fan causes the problem or the fan and heat sink assembly are cloggedwith dust.
Check the CPU fan to see if it spins freely, if not replace the fan.
If the fan and heat sink is clogged with dust the remove the dust and make surethe vents are clear and dust free. Use a small brush and can of compressed airto do this.

You more likely had a temperature issue with the CPU.
Had the CPU fan started to sound like an airplane warming up?
If so, you may have finally burned out the CPU.

Yes it's possible that it was reporting a problem with the power supply, but the only way to know for sure is to replace it.

In the meantime, eliminate everything else.
Disconnect power from EVERYTHING except the motherboard.
If it powers up, then it's possible that either your motherboard is failing (and now works under the decreased load), or one of the things you unplugged has failed.

If the unit springs to life, turn it off, and start reconnecting things.
If it fails constantly whenever 1 specific thing is plugged in, that thing or the motherboard chip that controls it is the problem.

Did you flash the main board ??

1.) remove battery
2.) press and hold the power button for 10 - 30 seconds
3.) replace battery.
4.) when the system boots it will say press XX (e.g F2) to load defaults and continue ,, press that key.

It could also be the power button is stuck!.

find were it connects to the main boad and unplug it
bridge the 2 pins it jacks into to turn the computer on. if it works u will need to replace that button or the case.

In G5 always the cover should be closed properly if not it will start the fan very fast like a blower and the red leds will be on showing that the heat is over so please check that the cover is closed and locked from behind so itworks smooth and the power supply is standard or normal
u should always use genune parts with mac..
thanks

Hi steve, PC repair is trial & error. I suggest you replace the power supply or test the power supply in another PC that also uses 4 SATA drives & two PCI-E slots for starters. Two boards with the same problem is odd. You must have the CPU & fan connected, memory installed, ATA 24_pin main power plugg connected (Page 17 in manual). 4-pin_12v power connected which is located (Just to the left of DDR1 memory slot). Looking at you're board the power supply should be no less than 500w.
CPU must have thermal grease between CPU & heat sink.
Even though you have fans running & LED's on & flashing, the power supply seems to be the first item to check. Make sure the bottom of the motherboard is not touching the PC case.
Try the above & post you're finding here.

Good Luck!
Mike

This happens because your CPU is overheating, open your PC and blow any dust off the openings and CPU/Power supply fans, if you think one of the fans is not working... replace it.

I'm sorry to tell you but your cpu has probably fried. Your pc hanging a lot probably meant something was overheating and the fact that you say your cpu is not hot when you have the cooling fan off of it means it is probably the cpu. When you first turn your system on without a cpu cooling fan, the cpu should be slightly warm, although I do not recommended running a cpu at all without a cooling fan or even touching it when it has power.

Either the new power supply you purchased is not functioning correctly. But, assuming it is now that you are getting some form of power to your PC, it is very likely the motherboard. Whatever fried your old power supply (or it could have just been the power supply itself) most likely fried your motherboard in the process. Probably not the CPU since the MB can't even post.

when you say "the fan makes a noise like its getting ready to spin", which fan...power supply's or cpu? if you answer power supply, then the POWER OK signal is not returning to power supply and it then quickly shuts down to prevent further damage. double check a firm connection to mother board. If you answer CPU, then it becomes far more complicated. Try re-seating the memory chips.