Samsung Galaxy S4 Volume Up button "stuck"

[WolfX4]

My grandmother dropped an S4 in water 2 months ago, I got it the other day and upon turning it on, the goes straight to the boot menu, unless I hold down the Volume Down button immediately after it vibrates, then it loads perfectly fine, and when it's fully loaded (you see icons, the time, etc.), the Volume Up button doesn't do anything, but the Volume Down works just fine. I took the phone apart, the button isn't physically stuck, and I used a voltage detector to see if a sign passes when I press the button down, and it does. Any ideas on what component is affected or a solution?