What was found
DURING CRUISE AT 43000' A RED DOOR PAX OPEN CAS MESSAGE APPEARED. RAN QRH & MESSAGE DISAPPEARED BEFORE COMPLETION OF THE CHECK LIST. VISUALLY INSPECTED DOOR & DOOR LOOKED NORMAL. THE CAS MESSAGE RETURNED & DISAPPEARED ALMOST IMMEDIATELY. LOOKING AT THE SYSTEMS PAGE, PAX DOOR SYNOPTIC WOULD CONTINUALLY FLASH RED ON & OFF, AFTER A FEW MINUTES, CAS MESSAGE RETURNED. LANDED IMMEDIATELY. DURING THE DECENT, CAST MESSAGE DISAPPEARED & EVERYTHING SEEMS TO INDICATE NORMAL WHILE ON THE GROUND AFTER POST FLIGHT INSPECTION. COMPLIED WITH OPS CHECKS IAW FIM OF CABIN DOOR OPEN & CLOSED. OPS CHECKS GOOD. COMPLIED WITH PRESSURIZATION OF CABIN TO CONFIRM THAT WE COULD NOT GET CAS MESSAGE WITH CABIN PRESSURE. ALL OPS CHECKS GOOD. ALL WORK IAW AMM TASK 21-31-00-860-803-A & FIM TASK 52-71-00-810-801-A, SUBTASK 810-005-A