What was found
WHEN F/O TURNED OFF F/D ALTITUDE HOLD, RECEIVED AN INTERMITTENT FIRE WARNING. THE F/O TAPPED THE GLARE SHIELD AND THE FIRE WARNING LIGHT AND BELL CAME ON STEADY. ENGINE NR 3 WAS SHUTDOWN IAW PROCEDURES AND UNEVENTFUL. SIMULATED CREW ACTION BY TAPPING ON GLARE SHIELD-NO DEFECTS FOUND. CHECK RESISTANCE OF BOTH A AND B LOOPS-NO DEFECTS FOUND. RAN NR 3 ENG AND CHECKED DUCT LEAKS, NO DEFECTS FOUND. AS PRECAUTIONARY REMOVED AND REPLACED NR 3 ENG FIRE DETECTOR CONTROL UNIT. OPS CHECK GOOD. REF MM 26-11-21.