Apple Diagnostics is a tool you can use to diagnose problems with your computer's internal hardware, such as the logic board, memory, and wireless components. You may be able to start your Mac with Apple Diagnostics, even if it doesn't start using macOS.

What does Apple Diagnostics tell you?

Apple Diagnostics is a built-in test that helps detect what hardware components are causing the problem, and find system failures. The test checks RAM, power controller, logic board, battery, and other wireless modules. ... When referring to Mac hardware test, you might also hear about Apple Hardware Test (AHT).

How do I get rid of Apple Diagnostics?

"Stop Testing" button at the top or "Restart" and "Shut Down" buttons at the bottom of Hardware test window. If you see these buttons, click the appropriate one.