As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 718 - EN ROUTE TO MKE AUXILIARY POWER UNIT FIRE WARNING BELL SOUNDED, NO LIGHT ILLUMINATED, ALL OTHER INDICATORS NORMAL. BELL WAS CUTTING IN AND OUT. REMOVED AND REPLACED OPERATING POWER UNIT FOR AUXILIARY POWER UNIT FIRE DETECTION SYSTEM. SYSTEM OPERATES NORMALLY. (M)