Follow the steps to run a diagnostic on Mac Silicon: Start up your Mac. Continue to press and hold the power button as your Mac turns on. Release the power button when the startup options window appears. Press Command-D to run the Diagnostics. Apple Diagnostics will run automatically. When it is complete, it will present a list of the problems encountered. Steps to run a diagnostic on Mac Intel Processor: Start up your Mac and press and hold the D key while it turns on. Release the key when the progress bar appears or when you're asked to select a language.