Running Apple Service Diagnostic from USB boot thumb drive

Hi Apple Community,

I have a mac pro 3,1 I am not yet ready to consign to the dump as it is clean and runs well except for the frequently reported black screen crash and fans on full problem - after anything between two minutes and 2hrs or so (record recently is 2:45hrs)

I intend to use the ASD-3S123 (Apple Service Diagnostic tool specifically for the Mac Pro 3,1 - which I have created on a bootable USB with the suggested EFI and OS boot partitions) using the helpful advice here https://www.makeuseof.com/tag/apple-service-diagnostics-mac/ - and verified from other qualified sources keen to assist whilst we are in lockdown due to Covid restrictions.

(somehow it doesn't feel right to jump in the car for a two hour drive to a different area to hand it over to a service technician and put others at risk right now, and fortunately others seem to agree and have been very helpful - some things are more important than fixing a Mac!).

So, I want to more thoroughly diagnose (what I suspect to possibly be) a logic board / logic board component issue(s). I have followed all other routes of troubleshooting including trying a known good replacement PSU.

Buy my issue currently is that it appears I cannot get the Mac Pro 3,1 to boot from that USB

(this is made trickier by the fact that the GT 9500 GPU is not flashed so I get no bootpicker screen, and am therefore 'driving blind'). I tried removing all other drives from the Mac so that the usb stick is the only option but no amount of L/R arrow and pressing return key seems to get it to go into the ASD (or if it does, then the graphics card is not able to display graphics whilst in this mode - I guess because no KEXT exists on the bootable ASD drive for it?).

N.B. I have verified that I can boot from this USB drive on other macs (although of course it would be the wrong set of ASD tools for those other machines)

Any ideas ?

Does anyone know what order the EFI and OS partitions present in (on the bootpicker screen for selection) - I think I need to boot the EFI partition ?

thanks

Andrew