How to Start the Diagnostic Mode on a MacBook Pro
Intel-based Macs, such as the Macbook Pro, have a diagnostic mode that checks the computer's hardware components for any signs of failure. You can use the diagnostic mode to check your hardware and eliminate it as a cause of problems if your system is exhibiting problematic behavior. Depending on your needs, select basic or more extensive testing.
Remove any external peripherals from your Macbook Pro, such as mice, keyboards or monitors.
Restart the Mac laptop.
Press the "D" key as your computer boots and continue holding it until you see a gray start screen.
Choose your language and press right. Select the basic test battery by pressing "T" or perform the more extensive hardware test by checking "Perform extended testing."
Click "Test." All test results appear on your Macbook Pro's screen.