What was found
DURING INITIAL CLIMB, THE CREW RECEIVED A NR 2 ENGINE FIRE WARNING. ENGINE INDICATIONS WERE NORMAL. THE CREW INITIATED AN IN-FLIGHT SHUTDOWN OF THE NR 2 ENGINE IAW THE QRH. THE CREW DECLARED AN EMERGENCY AND RETURNED TO LAS. THE PASSENGERS DID NOT BRACE. MAINTENANCE DISCONNECTED THE FIRE LOOP CONNECTORS AND PERFORMED A MEGGER CHECK OF THE FIRE LOOPS. A & B LOOPS MEASURED 1.5 K OHMS AND OPEN TO GROUND. MAINTENANCE THEN PERFORMED A MEGGER CHECK ON THE HARNESS TO GROUND AND FOUND NO FAULTS. AS A PRECAUTIONARY MEASURE, MAINTENANCE REPLACED THE FIRE DETECTION UNIT IAW MM 26-12-34 PG 401. OPS CHECK GOOD.