Last week, and then again last night, I restarted after applying a software update to my cMP (last night it was a LittleSnitch updater that requires a restart) and it was very sluggish to get to the startup chime. Last night on a second restart there was simply no startup chime at all. Primary cMP - 12-Core X5680 - see my sig for the rest - this is my main machine. I have also have a backup, secondary cMP - Hexa-core X5680 - used as a media and internet server. Fans run, and optical drive and hard drives spin up. I'm doubting it's a PSU issue. No LEDs indicating on the CPU tray. Here's what I've tried already - sorry it's going to be a little verbose: 1. Took out ram from CPU-B - restarted - No Sign Of Life (NSOL) - fans run, but don't roar. 2. Put ram back into CPU-B, and removed ram from CPU-A. NSOL - fans run, but don't roar. 3. Replaced PRAM battery - CR2032. Pressed the button to the left of the PRAM battery. NSOL - fans run, but don't roar. 4. Unplugged the power cord, and held down the power button for 10+ seconds. Booted back up - NSOL - fans run, but don't roar. 5. Removed/disconnected all drives and PCI-e cards. Try to boot up from Firewire external drive. NSOL - fans run, but don't roar. 6. Placed my CPU tray into my other cMP 2009 (Hexa-core - single CPU). NSOL - fans run, but don't roar. 7. Swapped in my ram from my Hexa cMP to my 12-core - NSOL - fans run, but don't roar. 8. Remove CPU-A, inspect - no obvious signs of damage - clean CPU and heatsink, apply Arctic Silver5. Reinstall, restart - NSOL - fans run, but don't roar. 9. Remove CPU-A CPU and remove Northbridge heatsink. Inspect - no obvious sign of damage - both push-pin rivets in excellent condition - clean heatsink and Northbridge chip thingy, apply Arctic Silver5. Broke one of the push-pin rivets, so replace it with one of the push-pins I bought off eBay (had to cut 3mm off the top - used the original rivet for the one that hides under the CPU heatsink, and the replacement one that is exposed). Restart - NSOL - fans run, but don't roar. 10. Remove, inspect, clean and re-install CPU-B. NSOL - fans run, but don't roar. 11. Put my 12-core tray back in my hexa-core cMP to see if that changes anything - nope - NSOL - fans run, but don't roar. 12. Put the 12-core CPU tray back in my 12-Core cMP. Get a red LED Ram indicator on ram slot 7. cMP starts up and boots to Recovery HD. Fans roar. Hopes are high, but on restart NSOL - fans run, but don't roar. 13. Put Hexa-core CPU tray into 12-core cMP. Boots fine - all ram shows up. Fans roar but then settle down after a minute or two. Restart - same result. Shutdown - put 4 x 8GB ram modules from my 12-core CPU tray into the Hexa-core tray - restart - no problems all working. Final result - the Hexa-core CPU tray is now in my primary cMP and working fine with all my drives and PCIe adapters etc. So, what I learned: Can't be a PSU issue - Hexa-core CPU tray boots fine in my primary cMP. Ram not an issue - works in my Hexa-core CPU tray in my primary cMP. New PRAM battery - so that's not an issue. All my hard drives, PCI-e cards etc are all working in my primary cMP when running my Hexa-core CPU tray. So I'm guessing it's something on the 12-core CPU tray. When I press the Diagnostics LED (on the backplane, to the left of the PCIe fan): I get a green LED for the third one down that has PSU PWROK to the right of it. I get an orange LED for the 4th one down that has 5V STBY to the right of it. I also get two green LEDs to the left of the DIAG LED button - one is the top one with SYS PG to the left of it, and a green LED for the GPU OK. The only issues I've had leading up to the failure have been the long wait for the startup chime (bong) - temps have all been around average - no crashes or unexpected shutdowns. Any ideas on what the issue might be? Any other troubleshooting ideas I can try?