Hi @danj – I'm also experiencing a similar issue on my MacBook Pro 15" Retina Display Mid 2012 (Mojave).
I've followed the steps you've listed above to diagnose my issue but haven't been able to figure out exactly what's causing my MacBook Pro to heat up/fans run without high CPU usage.
Here's what I've done so far:
–tried to get to the onboard diagnostics but no luck. I get the same error code (0x8000000000000003, Cannot Load 'EFI/Drivers/TestSupport.efi)
–got rid of auto loading apps
–no longer use Chrome (so sad)
–checked out the size of my drive and available space (698GB available of 751GB)
The link below is a little snapshot of what I'm experiencing on my MBP
–link: https://youtu.be/9SPHl8erWIg
I'm hoping to use this MBP for personal projects that require Powerpoint, Excel, Photoshop, etc. so fingers crossed the issue im experiencing is a simple fix (i.e. thermal paste, fan speed control, etc.) instead of having to get a completely new laptop.
Thanks in advance!