As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMB OUT AT 4,000 FT THE DIFFERENTIAL PRESSURE INDICATION DROPPED FROM 1.6 TO ZERO IN AUTO PRESSURIZATION MODE. THE CREW DID NOT FEEL A CHANGE IN PRESSURIZATION AND DID NOT SWITCH TO MANUAL MODE. THE FLIGHT LANDED BACK AT DEPARTURE WITHOUT INCIDENT. MX RESET THE PRESSURIZATION SYS AND THE PRESSURIZATION SYS OPERATED NORMALLY IN AUTO AND MANUAL MODES.