Apple Diagnostics for Mac computers

Overview: How to run Apple diagnostics on an Apple Mac mini or laptop

Official instruction here: https://support.apple.com/en-us/102550

Step 1a: Hold power/touchID for 20 secs

Ensure the Mac mini or laptop is powered OFF. Hold the Touch ID (or power button on Mac mini) for around 20 seconds until the options menu appears

Step 1b: Hold Command ⌘ +D until Diagnostics load

Hold Command (⌘) + D on your keyboard until the Diagnostics loads, which could take up to 60 seconds

Step 2: Run Diagnostics offline

After selecting language and agreeing to Apple's T&Cs, choose to Run Offline Diagnostics

Step 3: Record result code, Shut Down

Wait until the Diagnostics have completed. The Diagnostics will always return a result code, even on healthy hardware; please note the result and pass this info to your support team, then Shut Down the system.

Further troubleshooting: Boot from USB stick

There are some situations, such as a corrupted disk, where the above Diagnostics will not run. In such situations it is possible to create a USB stick which will boot and run the Diagnostics - this can even retore a corrupted disk back to a working state so may be worth the effort. Detailed instructions are available from Apple, or ready to download from 3rd-parties such as here: https://github.com/upekkha/AppleHardwareTest

Use another (working) Mac to recover even from bad firmware

How to revive or restore Mac firmware: See official Apple guide at https://support.apple.com/en-gb/108900