One Corner Of My DualSense Touchpad Does Not Respond To Touch

The detail that makes this diagnosable is that the click still works in the dead corner. Those are two separate systems and you have isolated one of them.

The click is a physical switch under the whole touchpad assembly, which is why it works everywhere regardless of the touch surface. Touch detection is a capacitive grid printed on the pad, read through a ribbon cable to the board. A region of the grid going dead points at one of three things: the ribbon not fully seated, a crack in a grid trace, or a damaged section of the capacitive layer. The fact that it is a contiguous region rather than scattered dead spots is characteristic of a trace or a connection problem, because the grid is read in rows and columns and losing one connection takes out a whole band.

Start with the ribbon, because it is free and it is the most common of the three. The touchpad ribbon on a DualSense sits under the board and is easy to disturb, so this is particularly common on controllers that have been opened before, or that have taken a knock. Reseating it properly, fully home and squarely in the connector, resolves a meaningful share of partial touchpad failures.

If reseating does not fix it, the touchpad assembly is a replaceable part and it is not expensive. It is a slightly more involved repair than a stick module because the ribbon routing is fiddly, but there is no soldering.

One thing I would not bother with: there is advice circulating about pressing firmly on the dead area or warming it. Neither does anything for a trace fault and pressing hard on a capacitive layer can crack more of it. If the ribbon is seated and the region is still dead, replace the assembly rather than trying to persuade it.