BMBT3DS is a bare metal button testing .firm payload for the Nintendo 3DS. It can be used to test all buttons on the console, and to see if one is being held down or not.
The latest release of bmbt3DS (the bmbt3ds.firm file)
Section I - Preparing
Power off your console
Remove the SD card from your console and insert it into your computer
Copy bmbt3ds.firm to the root of your SD card
Rename boot.firm to luma.firm
Rename bmbt3ds.firm to boot.firm
Safely eject the SD card from your computer and reinsert it into your console
Section II - Running the test
Power on the console without pressing any buttons
This payload does not show anything on either screens until a button is pressed. Refer to the attached image for what parts of the screen will light up for each button pressed.
Press each button once, verifying that the corresponding button shows up on the top screen. If it does not show up, the button pressed may be faulty. If you see a constant white square for one button and you are not holding the button down, that button may be stuck and could be the source of some issues.
Once you are finished with the button tester, press the power button once to power off the console
Remove the SD card from your console and insert it into your computer
Delete boot.firm from the root of your SD card
Rename luma.firm to boot.firm
Safely eject the SD card from your computer and reinsert it into your console
Power on your console
If all went well, you have successfully tested the buttons on your console.
