Run diagnostic on MacBook Air (M1,2020) with broken screen

Connect an external display. Use Command-F1 to toggle mirror mode so the external display is the main (not extended) desktop.

AST2 is the only option for Macs for many years now.

Good luck getting the diagnostics to work when the built-in display is not working. Even if you connect an external display you may discover that the video will not be shown on the external....even if you were to disconnect the built-in Display Assembly. Some boot screen items are now hardwired to only show on the built-in display.

At best you can try to time your key presses to hopefully trigger the necessary events to allow the broken M-series Mac to reach the actual Apple Diagnostics screen. It is not easy. I have done this multiple times, but I still cannot recall the exact timing. With the laptop completely powered down (yeah, good luck being sure this is the case since there are no trustworthy external signs these days), hold the power button for about 10 seconds or so to trigger the "Startup Options" boot process. Then give it some more time.....I think about 15-20 seconds, but you can wait longer to be safe. Then press & hold Command + D to load the diagnostic mode....hold for about 10-15 seconds (laptop reboots for this). Then wait again (not sure how long). The press the "Return" key to agree to the EULA or message about Diagnostic Mode. Wait some more (not sure how long), then press the "Return" key to agree to the next item. After which you should see the laptop show up in the GSX Diagnostic Console window with a solid blue bar (this can take 30 seconds or so sometimes).

I highly recommend testing with a laptop which has a working screen to figure out the time between each part of the boot process.

I wish you luck with repairs, they will be a pain since Apple seems to be going out of their way to make them as annoying & difficult as possible these days.

It is usually easy to get the Intel Macs into Diagnostic Mode even with a broken screen since you can start off blindly with a specific key press (Option + D) especially if you are using a wired Ethernet connection. You just hold the keys a bit, then release & wait a while. Then blindly press the "Return" key once and wait about 15 seconds or so and press "Return" again. At that point the Intel Mac should show up in the Diagnostic Console within about 30 seconds or so if everything went well.

ASU is only needed for a few specific cases with the T2 Macs (I forget exactly when ASU is required...it is a pain because Apple forces you to be running on the host Mac the absolute latest OS which is now Sequoia). And the ASU app gets in the way if you are also using the host Mac for other things even for DFU Mode (Finder or Apple Configurator).

I'm glad you were able to figure it out. Unfortunately you will run into a lot of situations where diagnostics won't work correctly and trying to determine the source of the problem will be a pain....is it the non-working computer, your host Mac, your local network or ISP, or is it GSX, or Apple servers causing the problem. Many times you will never know and Apple usually is no help as they will blame your local setup since there is usually no way to troubleshoot each individual component of the local setup (Hint, most times the problem is on Apple's end, but they are unaware or won't admit it). Pay close attention to how everything works so when something goes wrong you may be able to tell at what point it fails. Or in this case, how to boot blindly into the various modes.

It used to be much better years ago, but everything has been getting progressively worse over the years and no one at Apple cares to take action constructive feedback to address the issues (they play lip service to accepting feedback and the contact may actually care, but whatever manager in charge receives it doesn't act on it). The diagnostics used to allow you to select & deselect individual tests, and allowed you to run them in loop mode for 24 hours, now the diagnostics for the M-series Macs complete in minutes with the longest taking only about 10 minutes. The diagnostics are now basically worthless for troubleshooting and are only meant to be a required step for ordering parts & finalizing completed repairs. You would do well to try & develop some other methods for troubleshooting these Macs....I still don't know of any good way to do this on the M-series Macs since all my old utilities won't work on the M-series Macs.

FYI, keep article# 106048 bookmarked to review the list of known diagnostic issues....the list is very long and embarrassing.