What was found
DURING DESCENT, PASSING THROUGH FL410, CABIN OXYGEN MASKS DEPLOYED. THERE WAS NO WARNING OR INDICATION. CABIN PRESSURE WAS FINE. AFTER MASKS DEPLOYED, CREW GOT THE NORMAL INDICATION WHICH WAS A CAS MSG THAT "PAX OXY ON". REPLACED ELECTRO-PNEUMATIC ACTUATING VALVE (EPAV) AND TESTED SYS WITH ALL NORMAL OPERATION. BUT, THE REPLACEMENT VALVE'S 'SHOP FINDING REPORT' FROM STATED THE VALVE WAS INSPECTED AND TESTED WITH 'NO FAULT FOUND' AND HAD A HISTORY STATING THE PREVIOUS REASON FOR REMOVAL WAS AN INTERNAL OXYGEN LEAK INTO THE PAX SYS CAUSING DEPLOY MODE. NO CONFIDENCE IN REPLACEMENT VALVE (SN 218) SO ANOTHER REPLACEMENT TO BE INSTALLED.