I was working on my dual CPU Mac Pro 4,1 (upgraded to 5,1), having removed the CPU tray to access the board behind it, and subsequently the machine was non-bootable. Following the process of building the machine back up from zero has shown the machine to be bootable when only a single stick of RAM is present for each CPU socket. Adding a second stick to each CPU causes the CPU board to light up the red LED(s) shortly after powering on, and CPU A gets very hot. (Before I opened the machine it booted fine with three sticks per CPU.)
After failing to boot, I removed the CPU tray again and noticed what looked like a dust bunny in the processor tray socket. I tried using some compressed air to remove it, and while I can no longer see it, it hasn't helped.
Any suggestions on how to resolve this? Thanks!