What was found
A RED O2 FAIL WARNING MESSAGE ILLUMINATED DURING THE FLIGHT. THE AIRCRAFT RETURNED TO DCA. OUTSTATION MAINTENANCE INSPECTED THE OXYGEN SYSTEM IN ACCORDANCE WITH DORNIER MAINTENANCE MANUAL 12-19-01. THE AIRCRAFT WAS RETURNED TO SERVICE. LATER THAT DAY, THE WARNING MESSAGE OCCURRED AGAIN. MAINTENANCE INSPECTED AND REPLACED THE AIR DATA COMPUTER, IN ACCORDANCE WITH DORNIER MAINTENANCE MANUAL 34-11-01. OPERATIONAL CHECKS, INCLUDING A FLIGHT CHECK WERE COMPLETED. THE RESULTS WERE SATISFACTORY, AND THE AIRCRAFT WAS RETURNED TO SERVICE.