can't get Apple Hardware Test to work

Apple Hardware test is for Macs introduced before June 2013.

The diagnostic is loaded from

System/Library/CoreServices/diagnostics ... directory

But unfortunately, most Macs that were shipped with that directory correctly loaded with the diagnostic have Erased that directory or Replaced the original drive since they were built. and there is no software responsible for putting it BACK.

It is still on the original release DVD disk 2, if you still have that.

You can also build a bootable diagnostic USB-stick from the correct member of the group of all Apple Hardware Test images collected by a User and posted on GitHub. (not illegal, they were all found readily-accessible.)

https://github.com/upekkha/AppleHardwareTest

.

You're out of luck, sounds like you applied the Firmware patches along about SecUpd 2019-002 in 10.13.6 that disabled Hardware testing.

Just for the record, I have not seen enough cases to be convinced that the firmware update has anything to do with not being able to do D-for-diagnostic.

Even if it directly causes the loss of "auto" D-for-diagnostic capability, most of these Macs do not have the diagnostic directory present any more, so they can not possibly run it that way anyway.

I'll be darned, just checked my installs all the way back to 10.6.8, no such directory, 10.12.6 did have diagnostics_agent at that location!

Could not run AHT 3A212 on this iMac 12,1 that had the Firmware update Boot ROM Version: 87.0.0.0.0 though.

Yes, the Normal Install will require the Firmware be updated, I just cloned a 10.13.6 Install from my iMac thereby avoiding the Firmware change.