‘D’ Key [Diagnostic]
If you keep press the ‘D’ key, it can show you the issue hardware
HOW IT’S WORKS:
In the scenario where your Mac is having issues, and you’re not sure about the error, and you’re not a technology expert, the first thing we recommend is to perform the official diagnostic provided by Apple.
How to perform this diagnostic? It’s simple! The first thing to do, once you have the Mac [in any of its versions] turned off, is to turn on the device. Immediately, you will hear the startup sound [indicating that power is generally reaching all circuits, and their peripherals are responding in the initial step]. Once you hear the startup sound, immediately hold down the ‘D’ key until the visual information changes and prompts you to connect to the internet. This way, the Mac will be checking all components to determine which of the verified components is causing the error.
Note: This diagnostic is accurate at the time of providing the response but may not detect all errors. There are other types of errors, which we call SILENT ERRORS, that can be resolved based on accumulated experience.
