Cannot Load EFI/Drivers/TestSupport.efi

Try booting into Safe Mode which disables any unnecessary drivers, Startup & Login items.

What is the exact model of your iMac? You can get this information by entering your serial number here or here.

You may have a failing hard drive. To check the health of the hard drive create a bootable Knoppix Linux USB drive using Etcher (Mac/Windows/Linux). Option Boot the Knoppix drive and select the orange icon labeled "EFI". Once at the Knoppix desktop click on the "Start" menu on the lower left of the Taskbar and navigate to "System Tools --> GSmartControl". Within the GSmartControl app double-click on the drive icon which will show the health information & attributes for the drive. Post the GSmartControl report here using the "Additional Text" icon which looks like a piece of paper. Post the report even if the drive is listed as healthy as the report may contain clues. If your iMac contains a Fusion Drive, then post the report for both the SSD & the hard drive.

What version of macOS is on this iMac?

Hello again LadyJ59 🙄

Is THIS your computer type? https://everymac.com/systems/apple/imac/specs/imac-core-i7-3.4-27-inch-aluminum-late-2012-specs.html

Have you tried to start in Recovery Mode and re-install your operating system?

https://support.apple.com/en-gb/HT201314

Thank you, LadyJ59

Have you tried to start in Recovery Mode and re-install your operating system?

https://support.apple.com/en-gb/HT201314

Please click on the link to determine what you should do (in my opinion!)

Good luck!

D.

You cannot fix it. Apple has broken Diagnostic mode on some systems with the newer firmware updates on the older systems.

What problem are you having that you need to run the Apple Diagnostics?

Have you tried to start in Recovery Mode and re-install your operating system?

https://support.apple.com/en-gb/HT201314