If your buttons are sticking and they cannot be cleaned from the outside, here is how you can clean it from the inside.
No parts required.
To get into the controller, you will need to unscrew the 7 screws on the back.
With the screws removed, turn the controller upside down and separate the front end from the back end.
It is likely that the buttons will fall out of place, so as you open the controller, try to keep track of where the pieces go.
There should be the 4 circular buttons labeled 1-4, 4 smaller circular buttons for the D-Pad, and 3 rounded rectangular buttons.
Turn over the front side of the controller to remove all the buttons.
Wipe each individual button down with a piece of cloth and rubbing alcohol.
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