Chosen Solution

I recently upgraded my 5,1 flashed 2009 Mac Pro to dual 6-core X5680s as well as 128 GB of RAM. Since this upgrade I’ve encountered a few issues that oddly don’t seem to be causing any problems following boot up. Currently I’m dealing with a CPU diagnostic light that won’t go away no matter what I’ve tried. The strange thing is that when I power on the computer, the light shuts off and the computer proceeds to boot completely fine. I don’t see the OS throwing up any errors, it recognizes both CPUs are in place. I’ve moved RAM cards around, pulled the heatsink and reapplied thermal paste, and attempted an SMC reset but so far nothing has worked. Any help in figuring this out will be greatly appreciated. Edit: Both CPU LEDs are actually illuminated, I just didn’t notice the second one since it was obscured by the heatsink.

Alright final update for this “problem”, as this thing just continues to confuse me. I finished getting this rig ready for sale with the last round of upgrades and set it aside for photos. As I was intending on parting ways with this thing with the diagnostic lights still on, I photoed the thing running hooked up to a display to show the machine functions, except this was what surprised me: the diagnostic lights are gone. They flash briefly on startup and shutdown but they no longer stay on like they used to. I have absolutely no idea what caused this as the only thing I did was install a GPU and Mojave, let the thing sit without power for a day, then plugged it back in and viola! I’ve power cycled this thing several times since just to see if it decides to jack with me some more but so far it seems happy. Hopefully this escapade helps anybody who runs into a similar fluke, though I’m still unsure what started it in the first place. Cheers.

I would suggest booting into verbose mode to see if the boot process spits out any recognizable errors that you can search up. When it comes to any seemingly hopeless Mac issue, I go there. The only thing that comes to mind is maybe an EFI-related problem. Coming straight from the Mac Pro 2009 Technician Guide, the description of the CPU Diag LEDs states: The 8-core and quad-core riser boards include a diagnostic LED for each processor that you can use in troubleshooting processor issues. The following illustrations show the location of each LED and its associated processor. These error LEDs are normally off. A red illuminated LED indicates the associated processor may be missing or in overtemp state (PROCHOT). (Page 34 of the Tech Guide posted here) However, if you wanted to see if its truly a CPU related issue, it suggests: Determine if the error LED follows the processor heatsink; verify the heatsink is physically undamaged and properly connected to the processor board, check that adequate thermal grease is applied, and verify the fan is operating. For 8 core riser boards only: determine if the error LED follows the processor by swapping processor A and processor B locations. So, swap the CPUs into each others slots, see if the light changes to the opposite LED. If so, I’d guess it’s the CPU. If all else fails, I’d say maybe something went wrong during the flashing process and the EFI is damaged, in which case you’d have to retry that entire process. But, rule out everything else first. Read through the technician guide to see if you find anything else that could be an issue. Let me know if this helps, Alex