What was found
AIR TURNBACK, AFTER APU SHUT DOWN, APU FIRE WARNING BELL ACTIVATED, CK LIST WAS C/W AND WARNING WENT OFF, THEN CAME ON SEVERAL TIMES BEFORE LANDING. C/W APU COMPT INSPECTION, NO DEFECTS FOUND, DEFERRED APU PER MEL. LATER DURING RON AT OAK, APU FIRE DETECT MX FAULT TEST FAILED. REMOVED AND REPLACED APU FIRE CONTROL PANEL AND FIRE DETECT UNIT WITH NO HELP. REINSTALLED ORIGINAL EQUIPMENT, TESTED APU FIRE DETECT LOOP AND CLEANED LOOP CONNECTOR. OPS CK GOOD PER MM.