MP 6,1 Mac Pro 6,1 CPU upgrade- shutting off at login screen

[AWalkerStudios]

So I’ve got a 2013 Mac Pro that I just popped a used 12 core CPU into. It started up without issue, ran bench marks successfully and even had a few work hours logged on it. Then it just cut out and shut off randomly. I haven’t been able to get it to turn on past the login screen since. Any tips? I was going to try going back in and reapplying the thermal compound a little more liberally.
thanks

 

[MisterAndrew]

Have you looked over all the steps for replacing the CPU Riser Card, and prerequisites, in Apple's technician guide posted in this thread? https://forums.macrumors.com/threads/mac-pro-cylinder-led-diagnostics.2231524/

These are far more precision and sensitive than a typical desktop computer. Pay close attention to the torque values indicated. Apple says, "The indicated torque value must be used to avoid damaging the unit or causing the unit to malfunction." Avoid working on it further unless you have a proper torque screwdriver (Apple uses a Wera brand) and of course a functioning ESD wristband (I know that's obvious, but YouTube people often neglect that). There are a lot of small things to pay attention to that 3rd party guides & YouTube videos do not mention. For example, be careful to route the I/O flex cable exactly as shown in the technician guide under the I/O and Power Supply Assembly repair instructions.