Things You'll Need
Instructions
Close your Nintendo DS and place it on a flat surface so that the back of the console is facing up. Remove the tri-tip screws.
Remove the remaining screws with a Phillips screwdriver. Place all of the screws in a small dish or other safe place so that you do not lose them.
Lift the back panel from the DS and set it aside. Remove any dirt or other debris around the "L" button with a cotton swab or toothpick. Pay particular attention to the contact underneath the button.
Check the "R" button for any dirt buildup. Even if your "R" button is working normally, you should remove any dirt now to prevent issues later. When the buttons are clean, replace the back panel and secure it with the screws you removed earlier.
Test your DS by loading a game that makes active use of the trigger buttons. Press the "L" button to see if it is functioning normally. If you are still having issues, contact customer support.