What was found
AFTER RECEIVING RED CABIN ALTITUDE CAS MESSAGE. RECEIVED CABIN DOOR OPEN CAS AND CHECK DOORS CAS MESSAGES. UPON LANDING CHECKED ALL DOOR INDICATORS & IT WAS FULLY LATCHED. POWERED AIRCRAFT WITH BATTERIES AND POWER CART REF AFM. PERFORMED ARES DOWNLOAD AND SENT IN FOR REVIEW. THIS ENTIRE EVENT APPEARS TO BE RELATED TO THE VENT DOOR NOT EVER BEING FULLY CLOSED (EVEN THOUGH INITIALLY IT DID INDICATE IT WAS). CABIN NEVER PRESSURIZED, SUSPECT VENT DOOR WAS ACTUALLY AT LEAST PARTIALLY OPEN, BUT PROX SWITCH SHOWED IT CLOSED. RECOMMEND THE VENT DOOR RIGGING BE VERY CAREFULLY CHECKED, AND REPAIR AS REQUIRED. REMOVED MED DOOR STEPS & INNER SKIN AS REQUIRED REF MM 52-10-50 AND 52-10-14. PERFORMED GVI OF THE DOOR LATCH MECH, PRIORITIZING THE EVALUATION OF THE VENT DOOR. FOUND ALL SENSORS WITHIN LIMITS. NOTED WITH DOOR HANDLE IN CLOSED POSITION THERE WAS A GAP BETWEEN THE PRESSURE PREVENTION PANEL AND ITS SEAL. ADJUSTED THE RIGGING OF PRESSURE PREVENTION PANEL WITH THE SEAL EQUALLY ACROSS THE ENTIRE SURFACE REF MM 52-10-04. INSTALLED MED DOOR STEPS AND INNER SKIN AS REQUIRED REF MM 52-10-50 AND 52-10-14. HAD AIRCRAFT RELOCATED ONTO THE RAMP. POWERED UP THE AIRCRAFT ON APU POWER AND STARTED THE LH AND RH ENGINES IAW AFM. PERFORMED THE PRESSURIZATION CONTROL SYSTEM OPS CHK (ENGINE RUN) IAW MM 21-30-00 WITH NO DISCREPANCIES NOTED. SHUT DOWN ENGS & POWERED DOWN ACFT. C