EDIT : I just noticed on my last boot up that only 4 of my 8 RAM slots are shown in System Profile/Memory last Friday afternoon my 4,1>5,1 2009 8 Core cMP shut down and would not start up. No chime and also CPU B's RED LED was turned ON. No fans were working at all. 1. I removed the PSU and thoroughly cleaned & de-dusted it it 2. I cleaned the Northbridge heatsink and re-applied Arctic Thermal Ceramique 2 & while I was at it I replaced the Northbridge rivets and also added a non-conductive washer under the rivets on the underside to give it a tighter grip ( similar to the nut & bolt technique. 3. I removed both CPU A & B - thoroughly cleaned and re-applied thermal. 4. I noticed that 4 of the pins on CPU A socket were bent. I used two magnifying glasses and non-conductive, clean, shaved to a fine point . . bamboo toothpicks ( I live in Japan ) to re-align the socket pins. 5. Installed only CPU A and after a few tries . . got it to boot up. with no RED LED's on the daughter board. ( currently everything non-essential have been removed. Just a HD 5770 + keyboard & mouse ) ============================================================ After a few sleepless days and lot's of PM help from h9826790 I finally have it booting again with neither CPU A nor CPU B showing RED LEDs + all fans are working normally . . . . but .. . . . . I have a total of 17Gb of RAM - the cMP came with 6 x 2 GB DIMMS all from the same batch/maker but only slots 1 & 2 on CPU A are recognizing any of the DIMMS. ( NOTE : I have only installed DIMMS from the original supplied 12 gb matched 2Gb DIMMS ) My question : Why aren't any of the DIMMs installed in CPU A nor CPU B ram slots 3 ~ 8 ( correct order/slot placement ) being recognized. Only DIMMS placed in CPU A 's slots 1 & 2 are recognized. NOTE : In installing CPU B I tightened the heatsink to "barely fingertight ".. and then one more full turn for each of the four bolts. How tight should the HEX bolts be ? I recall seeing threads regarding similar problems as I am experiencing but yet cannot find a definitive solution. Help, advice, solutions tremendously appreciated ! PS : Of course I have done both PRAM & SMC resets and I als oreplaced the motheboard CR2032 battery. ============================ UPDATE : Since my first post .. The Xeon 5570 in CPU B is not starting. The CPU B socket has NO bent pins. The PSU is OK. All fans spin up correctly including on CPU A & B. The Nortbridge heatsink is now "nut & bolt" attached to the tray with non-conductive washers top & bottom The Xeon 5570 itself appears to have no discolouration or visible damage. 1. I'm gradually coming to the conclusion that the Xeon 5570 that is currently in CPU B and was in CPU A at the time of the original shutdown failed for whatever reason - we had thunderstorms here at the time of the failure. 2. I have tried EVERY possible configuration - even removed most of the Blue Loctite from the CPU B bolt threads to give me a finer touch when tightening the Hex bolts. in the correct Apple designated order. 3. Thoroughly cleaned and re-applied Thermal paste to the CPU B Xeon 5570. ==================================================== The 4,1 starts up now with just CPU A. The fans do NOT ramp up their speeds. My Dual Quad Core 4,1 has reeverted to a "Single Quad Core". . I can install and see 10Gb of my original 17Gb RAM only if installed on CPU A side slots 1~3. Sys Info/Hardware shows only ONE processor which is in CPU A.