Precision 7920 wont POST

I have a Dell Precision 7920 that won't post (or do much else).

For a while we've had an issue where you had to press the PSU BIST button in order to get the server to power up but now it won't even boot. We have swapped the PSU and it still requires the BIST to be pressed.

Before pressing the BIST, the server flashes code 1-1 (Bad Motherboard) and after pressing the BIST the server has a solid white power LED as if everything is fine but we get no video ouput from our GPU and caps lock doesn't work on a keyboard.

If I remove the side panel then the intruder alarm goes off and the power LED flashes the fault code for side panel open which suggests some for of BIOS activity.

I have tried removing components and even swapping around our dual CPUs or even running on 1 CPU. Also when I remove all the RAM I don't get a flash code for no memory installed (which I did get a couple months ago when troubleshooting faulty RAM).

Does anyone have a suggestion for more steps to troubleshoot? or have encountered this before?