Is this for a
DFMEA or
PFMEA? I am guessing you meant DFMEA.
It would be difficult for a leakage test to prevent a design weakness. By the time you get to the point of testing, the design is nearly finalized, so this would be detection. An example of a preventative design control would be a finite element analysis of a design concept. This would allow you to revise the concept before the first prototype was built thus preventing the design weakness.