What was found
(CAN) LOSS OF COMM/NAV/ TRANSPONDER OCCURRED BACK IN NOV 2007. NOT REPORTED UNTIL 9/11/08 AFTER CUSTOMER HAD HEARD OF A SIMILAR OCCURRENCE ON ANOTHER CHALLENGER 300 (20016) THROUGH ADV COMMITTEE CONFERENCE CALL. IN FLIGHT CREW NOTED THAT TRANSPONDER CODE CHANGING FREQ. LT AND RT RADIOS INDICATED YELLOW ON C/P MFD. ALSO ON C/P MFD, HAD AMBER BOXED DCP FLASHING ON AND OFF. IAW RECOMMENDATION FROM REP, THE C/P DCP CHANGED OUT. TECHS UNABLE TO DUPLICATE PROBLEM.