What was found
RED WARNING CAS DOOR PAX OPEN CAME ON MULTIPLE TIMES WITHIN FLIGHT. ONCE AT FL380, ONCE AT FL400, BOTH TIME IT WENT AWAY WITHIN A COUPLE MINUTES. FOLLOWED THE QRH AND DESCENDED TO 10,000FT WHERE THE CAS MESSAGE CAME ON AGAIN. REMOVED AFT LOWER MED MICROSWITCH AND INSTALLED CUSTOMER SUPPLIED, NEW AFT LOWER MED MICROSWITCH. OPS CHECKS GOOD.