Will not boot, grey screen

"this sounds like a possible failed logic board or processor module. Since the 2.5 Quad is water cooled, first check the felt pad under the processor heat sinks - is it damp or stained? If so, stop trying to use the machine immediately - the LCS has failed!

Next, make sure you have a known good matched pair of RAM modules (these machine are VERY picky about matched RAM - try to use not only the same size, but the same manufacturer and code when possible) installed in the inner most slot of each bank - follow the diagram on the inside of the cover to ensure the RAM is installed in the proper banks.

Next, if the issue persists when you boot the machine after checking all of this, remove the cover and air dam and check the LED bank on the forward edge of the logic board, near the RAM slots:

http://support.apple.com/kb/HT2652

Red lights here are BAD (except for the ""cover open LED"" My guess is you will have either the ""Overtemp"" or ""Proc"" LED lit.

There are sources for the parts, let us know what you find..."

"remove as much RAM as possible,

unplug all unnecessary devices (USB/FW etc)

find and use CUDA button http://support.apple.com/kb/HT1939

If this not helps remove battery for some time (ie 1h) then place it again

If none of this help, I'm afraid that you have a hardware problem :("

In same old macs they didn't boot if the clock batery was dead, I don't know if it is the case but check the battery voltage just in case.

"From what you described my first guess would be the logic board. When you are in open firmware and the keyboard does not respond that could be a sign that the ports have stopped responding due to the failure.

My second guess would be the processors.

What I would try to do is try and start the computer using only 1 RAM slot. If you can try and boot to your system disc or your AHT (Apple Hardware Test) CD do that and see what happens.

If none of the above work, I think you'll have to take it to an Apple store to get it looked at.

good luck!"

"The powermac G5 is a very, very picky computer. you should try to limit the ram, check the case for leaking coolant, and other issues. it sounds like a logic board issue, try to reseat the graphics card(Unplug it than plug it back in while the computer is off) then torn the computer on, his might solve the problem. Try to also clean the contacts on the graphics card connector. Also, try to run the computer without the PRAM Battery, also try to run it without any drives connected, if you see a flashing ? folder, then your drives or cables may have failed. if none of this solves the problem, the logic board is most likely dead. if you really want to take it to the next level, try to reseat the CPU's. this is a hard thing to do, because the liquid cooling unit has to be removed.

Good Luck,

Chris"

"My G5 had the same problem. After 4 years and a couple of months with no problem at all it suddenly stopped working. Restarting the Mac failed with the grey screen as mentioned above. After doing the usual to check if it was a software or a hardware problem I found useful information in the links below.

The problem may be caused by fractures in the solder joints between the powerpc processor and the logic board. This may be a design flaw in the otherwise great machines Apple produces. What is needed is a ""reflow"" of the logic board. Contact Jürgen Geisbauer of the German Firm ""Golytronic"" (www.golytronic.de) for a really brilliant suggestion (which is too good to give away here) to check that this is the case with your mac too.My Mac is now in Germany for a reflow. If this helps to bring my mac back to life I will of course keep you posted.

check out these links:

http://discussions.apple.com/thread.jspa...

http://discussions.apple.com/thread.jspa...

http://en.wikipedia.org/wiki/Reflow_sold..."