As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SHORTLY AFTER T/O, THE FLIGHT CREW REPORTED A PROBLEM WITH THE AIRCRAFT PRESSURIZATION SYSTEM. AIRCRAFT WOULD NOT PRESSURIZE IN AUTO OR MANUAL MODE. AIRCRAFT RETURNED TO THE FIELD LANDED WITHOUT FURTHER INCIDENT. DENVER MX PERSONNEL REPLACED THE CABIN PRESSURIZATION CONTROL UNIT IAW AMM 21-31-00 OPERATIONAL CHECK WAS CONDUCTED WITH SATISFACTORY RESULTS. AIRCRAFT WAS THEN RETURNED TO SERVICE.