What was found
AFTER TAKEOFF EN ROUTE LOST ALL FUEL QUANTITY INDICATIONS, TURNED BACK AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE R/R FUEL QTY INDICATION COMPUTER (FQIC) IAW AMM 28-42-34, OS CHK GOOD. FUNCTIONAL TESTING OF FQIC S/N 1772 PRODUCED NO FAILURES OR FAULTS. UNIT SENT TO TESTING FACILITY WHERE UNIT PASSED INITIAL TESTING WITH NO FAULTS NOTED. UNIT THEN FORWARDED TO EUROPE FOR IN DEPTH TESTING. PERFORMED A PRODUCTION ACCEPTANCE TEST, ENGINEERING ACCEPTANCE TEST, AN ENGINEERING PRODUCTION TEST ON SYSTEM RIG THAT INTERROGATED THE FAULT LOG, TWO SEPARATE 40-HOUR TEMPERATURE AND VIBRATION TESTS, AND FINALLY OPERATIONALLY GROUND TESTED ON AIRCRAFT, & NONE OF THE TESTS COULD PRODUCE A FAULT, UNIT RETURNED TO SERVICE.