What was found
EN ROUTE ACA-ORD, DURING CLIMB, RECEIVED INTERMITTENT APU AURAL WARNING, NO LIGHTS. WARNING STOPPED BUT AFTER LEVEL OFF AT 29,000 FT WARNING STARTED AGAIN. RECEIVED RT ENGINE FIRE WARNING (AGAIN NO LIGHTS). LANDED MEX WITHOUT INCIDENT. INSPECTED AND CHECKED ENGINE AND APU FIRE DETECTION SYSTEM, NO FAULTS NOTED. REPLACED CENTRAL AURAL WARNING UNIT. SYSTEM GROUND CHECKED NORMAL OPERATION. (M)