As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - FLT NR 1535 - CLIMBING THROUGH FL 310, CABIN ALTITUDE WAS CLIMBING AT 1000 FPM. FLIGHT CREW WAS UNABLE TO CONTROL CABIN PRESSURE IN AUTO OR STANDBY. RETURNED TO PHL AND LANDED WITHOUT INCIDENT. PERFORMED PRESSURIZATION CHECK. SYSTEM CHECKED NORMAL. UNABLE TO DUPLICATE ANY DEFECT. (M)