As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, UNABLE TO PRESSURIZE; RUN CHECK LIST TWICE, NO RESULT. RETURN TO MPTO. SYSTEM WAS VISUALLY INSPECTED AND PRESSURIZATION CONTROL SYSTEM TEST PERFORMED UNTIL 4 PSI OF DIFFERENTIAL PRESSURE FOUND OK AT THIS TIME IN ACCORDANCE WITH M/M 21-00-00 PAGE 511.