What was found
DURING PRE-FLIGHT TEST, ENGINE #1 FIRE AND OVERHEAT DO NOT TEST IN NORMAL. WHEN SELECTING "B" LOOP #1 ENGINE IMMEDIATELY INDICATES A FIRE AND OVERHEAT. ENGINE #1, #2 , AND APU ALL TEST NORMAL ON LOOP "A". GROUNDED - YES. CONFIRMED FAULT, #1 ENGINE B-LOOP INTERMITTENTLY FAILS TEST. REF B737-900 AMM 26-11-00-730-801, FOUND DEFECTIVE MW0326 CORE FIRE DETECTION HARNESS. REMOVED AND REPLACED #1 ENG MW0326 FIRE AND OVERHEAT LOOP REF AMM 26-11-02.