The P is an abbreviation for "Progress," not an abbreviation for "Pass." It is a temporary grade. Grades of P (except for research and thesis within the program) must be changed before a degree or preliminary exam warrant can be issued.
If a P is assigned for research courses numbered 790, 890, and 990, the grade will remain until the faculty member assigns a grade of S or U. All previously assigned P grades in that instructor's research course will revert to an S or U upon assignment of the S or U grade. Usually this is upon assignment of the final grade for the course in the semester of submitting the dissertation. (All grades for 790, 890, 990 that are unreported by the end of the grading period will be automatically assigned a P.)