What was found
AIR TURNBACK. FLIGHT CREW REPORTED PRESSURIZATION SYSTEM WARNING HORN SOUNDED AT 14000 FT AND OXYGEN MASKS DROPPED. AIRCRAFT RETURNED TO DEN WITHOUT INCIDENT. PERFORMED OPERATIONAL TEST OF PRESSURIZATION CONTROL SYSTEM PER MM 21-31-0 AND FOUND NO DISCREPANCIES. SERVICED CREW OXYGEN PER MM 12-15-21. REPLACED PASSENGER OXYGEN BOTTLE PER MM 12-15-21. CLEANED AND STOWED CABIN OXYGEN MASKS PER MM 35-21-59 AND 35-21-52. SERVICED THREE PORTABLE CABIN OXYGEN BOTTLES PER MM 12-15-21. AS A PRECAUTIONARY PROCEDURE, PRESSURIZED AIRCRAFT TO 4 PSI DIFFERENTIAL PRESSURE WITH NO PROBLEMS NOTED.