As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED AIRCRAFT WOULD PRESSURIZE ON TAKEOFF UNTIL MAXIMUM DIFFERENTIAL WAS REACHED AND SAFETY VALVE WOULD ACTUATE. PILOT HAD NO CONTROL OVER RATE. GAINED ACCESS TO OUTFLOW VALVE AND FOUND SCREWS HOLDING THE DIFFERENTIAL SECTION OF CONTROLLER PULLED OUT OF VALVE ALLOWING SPRING TO UNSEAT THIS SECTION. SCREWS WERE NOT LOOSE, BUT HAD PULLED THE THREADS OUT OF THE PLASTIC BODY OF VALVE.