Greetings, in 2006 I bought a PowerMac G5 Quad (Costs: About 3500 $). Now in 2009, I suddenly experienced severe problems using the machine. When I turned it on, after 4-5 minutes the OVERTEMP LED came up and 2-3 minutes later the whole computer would go nuts and crash. After endless calls with Apple "Care" (funniest thing I've ever experienced. They wanted me to pay 1500 USD to get a replacement kit. Sorry, but they seem to be nuts - I could have bought a whole new quad or even a used mac pro for that money) I decided to disamble the quad myself. It really takes some time to disamble this CPU beast, you just have to loosen about a zillion screws to get it out of its cage. After several tests without luck I noticed that the CPU B cores get really hot in IDLE Mode (about 85°C). When the complete disambly of the CPU Unit didn't bring any luck, I decided to completly remove the unit. And guess what? Overtemp LED was gone - everything worked fine again. Now I have a Dual G5 2.5 GHz. Before I removed CPU B, I touched the radiator i noticed that the lower end of it (where CPU Core B is located) isn't even handwarm, while the top end is really hot. (I think those are two separated circuits). My conclusion: The cooling pump of CPU B bit the dust. No circulation equals very hot CPU equals OVERTEMP equals LED806 Checkstop (CPU hang) As I don't want to pay 1500 USD for a thing that may cost about 20 $ in production, I tried to find that pump on the Internet. And guess what? Success! I found this pump that looks very similiar to that on the quad: http://www.laing.de/produkte/pumpen/gleichstrompumpen_ddc/ (Sorry, the page is in German - I'm from Austria) I think I'll give this thing a try, since it costs about 70 euros which is A LOT cheaper than a new CPU Unit or even a Mac Pro. My questions is now: Has anybody ever done such a repair? I know it won't be easy, but i think it would be nice if my quad could be back to full power. Does anybody know what cooling liquid Apple is using? I think during the repair I won't be able to recover all of the liquid, so I may be forced to refill the unit. Is the radiator divided into two sections or is there only one? (Two or one circuits?) Thanks, I'll let you know if I make any progress by updating this thread.