Down On My D-Pad Does Not Register But The Other Three Directions Work

A D-pad is very repairable, so this is not a new controller situation. The symptom actually points to a specific and cheap part.

On this controller the D-pad is a single plastic cross sitting on a rubber membrane with four conductive domes underneath, one per direction. Nothing registering at all on one direction, from any pressing angle, means that dome's contact is no longer completing the circuit. On a two-year-old heavily used pad there are two ways that happens. Either the carbon pad on that dome has worn through to the point of not conducting, or the rubber dome has torn or collapsed so it no longer travels far enough to touch the board. Down is almost always the first to go on a fighting game pad for the obvious reason that it carries more presses than the other three combined.

The reason this is good news is what it rules out. A dead board trace or a failed controller would not spare the other three directions sharing the same connector, and a firmware problem would not single out one direction. So the fault is confined to a few pounds' worth of rubber.

Clean it first, since residue can bridge or insulate a contact and it costs nothing. If the direction is still dead after cleaning, the dome is physically gone and you need the membrane. Buy the full D-pad membrane rather than attempting a repair on one dome, because conductive paint patches on a dome do work briefly and then fail again, usually within weeks, and you will not want to open the controller twice. While you are in there, look at the D-pad cross itself: if the plastic under the down direction is visibly worn flat, replace that too, as it affects how far the dome travels.