What was found
DEPARTED SLC FOR OUR LIVE LEG HEADING TO JAX WITH ONE PASSENGER. UPON REACHING FL210 WE WERE PRESENTED WITH A RED ENTRY DOOR CAS MESSAGE. WITH PRESSURIZATION CHANGES BEING MADE DUE TO HE CLIMB, WE MADE THE DECISION TO RETURN TO SLC. AN EMERGENCY WAS DECLARED TO ATC IN WHICH WE STARTED A DECENT AND WORKED THROUGH THE QRH. UPON REACHING 9000FT, WE STAYED ON ATC VECTORS TO BURN FUEL TO REACH OUR SAFE LANDING WEIGHT.