1. Apple Silicon Macs:

   • Shut down your Mac.
   • Unplug all external devices except the keyboard, mouse, display, Ethernet connection (if applicable), and AC power.
   • Press and hold the power button on your Mac.
   • As your Mac starts up, continue holding the power button until you see the startup options window.
   • Release the power button.
   • Press and hold the Command (⌘)-D keys on your keyboard.
   • Apple Diagnostics will run and check your Mac for hardware issues. It will display results and reference codes¹.

2. Intel Macs (Released After June 2013):

   • Turn on your Mac.
   • Immediately press and hold the D key on your keyboard as your Mac starts up.
   • Release the key when you see a progress bar or are asked to choose a language.
   • If using the D key doesn’t work, try pressing and holding Option (⌥)-D at startup instead.
   • If neither method works, review the guidelines for using key combinations at startup.
   • You may be asked to choose a language and ensure an active network connection.
   • Apple Diagnostics will provide test results and reference codes².

3. Macs Released Before June 2013:

   • These Macs use the Apple Hardware Test instead of Apple Diagnostics.
   • To run the Apple Hardware Test, follow similar steps as for Intel Macs but use the appropriate key combination for your specific model.

Remember that Apple Diagnostics helps identify hardware issues, suggests solutions, and provides information on service and support options.