My MacBook Air doesn't start Apple Diagnostics
Following How to use Apple Diagnostics on your Mac - Apple Support, I turn on my MacBook Air by pressing the power button, immediately followed by holding down D. But instead of getting a screen asking you to choose your language, I get the chime, the startup Apple icon, and then the normal login screen. I'm holding D down the whole time.
The same thing happens if I hold down option-D.
What else can I do to run Apple Diagnostics?
MacBook Air, macOS High Sierra (10.13.6), MacBook Air 13-inch, early 2015