What was found
ON INITIAL APPROACH, FLAP SELECTED 20 WITH AMBER FLAPS FAIL CAS. FLAPS REMAINED AT ZERO AFTER SELECTING FLAPS TO ZERO AS DIRECTED BY QRH. CONFIRMED DISCREPANCY FLAPS FAIL IMMEDIATLY WHEN COMMANDED OUT OF ZERO WITH NO MOVEMENT. USED CHALLENGER FLAP FAIL TROUBLESHOOTING GUIDE FOUND ON SMARTFIX FOR NOTED CONDITION VERIFIED FOU POWER AND GROUND. NO DEFECTS NOTED. VERIFIED PDU POTENTIOMETER AND WIRING CIRCUIT, NO DEFECTS NOTED. VERIFIED FLAP CONTROL HANDLE AND WIRING CIRCUIT. NO DEFECTS NOTED.WHILE PERFORMING BDU CIRCUIT CHECKS WE SET THE FLAP BREAKOUT BOX IN "BOTH ON RIGHT" MODE. THE FLAPS MOVED TO 30 AND THEN FAILED. AFTER RESET WITH BOTH STILL ON RIGIIT THE FLAPS WOULD NOT MOVE EITHER WAY OUT OF 30 AND WOULD FAIL IMMEDIATELY WHEN COMMANDED. WE THEN RESET AND PUT THE BOX IN "BOTH ON LT" MODE, WE WERE ABLE TO MOVE FLAPS FULL TRAVEL UP & DOWN WITH NO FAILURE WE THEN SET THE BOX INTO NORMAL MODE AND AGAIN GOT FULL TRAVEL UP AND DOWN. WE THEN SET THE BOX TO "BOTH ON RIGHT" / AGAIN AND GOT FULL TRAVEL UP AND DOWN WITII NO FAILURES. WE REMOVED THE BREAKOUT BOX AND HOOKED EVERYTHING UP NORMALLY, WE CAN NO LONGER GET THE FLAPS TO FAIL SUSPECT INTERMITTENT RIGHT BDU. MCC REQUESTS REPLACEMENT OF RIGHT BDU. R & R RT BDU IAW AMM 27-51-07 PERFORMED OPERATIONAL TEST, NO DEFECTS NOTED.