These are not "retail and/or aftermarket" motherboards.

It's been my experience that usually, Dell-OEM motherboard only contain the necessary BIOS-firmware micro-code to boot-and-run the list of main-processors that originally shipped with it when new from the factory. Their PDF or marketing literature should have a list of CPU's.

@Tesla1856​ Thanks for your reply, and yes I agree, and this is what i have followed

All I am doing is upgrading from the entry level CPU (5945wx) to the top of the range (5995wx), this option that was available from Dell

@m74brown​ ,

Interesting.

So, I have also seen reports of newer revision motherboards. And while there is only one BIOS-Firmware available, they are hardware-different (obviously) with some slightly different BIOS options available. You can therefore extrapolate that maybe only they support the newer processors.

Are you at least running the latest BIOS-version?

@Tesla1856​ Yes, latest BIOS. And what makes me suspect the CPU in faulty or locked, is that my system was supplied by a Dell Partner as refurbished, the services tag of the system shows it left dell with a 5995wx CPU in it (providing the chassis and motherboard are the same) which I believe they are

@m74brown​ ,

@Tesla1856​

1. Yes, latest BIOS.

 

2. my system was supplied by a Dell Partner as refurbished, 

1. Cool, good.

2. The plot thickens ...