Shut down your MacBook Air
Press the power button, then immediately press and hold the D key
Release the D key when the Apple Diagnostics screen appears
Follow the on-screen instructions
Wait for the diagnostics test to complete
Note any reference codes shown
Restart your MacBook Air after the test finishes
If Apple Diagnostics does not start, shut down again and try holding Option-D instead
If using an Apple silicon MacBook Air, shut down, then press and hold the power button until startup options appear, then follow the on-screen diagnostics option if available
Connect to Wi-Fi if prompted during the test
Contact Apple Support if error codes are reported