As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON DESCENT, AFTER STARTING APU, CREW RECIEVED FIRE WARNING. CREW COMPLIED WITH CHECKLIST AND EXPENDED BOTH FIRE BOTTLES. INSPECTED AND FOUND NO EVIDENCE OF FIRE OR OVERHEAT CONDTION. REMOVED AND REPLACED APU TAILPIPE MUFFLER, AND FIRE EXTINGUISHER BOTTLES PER BOEING M/M.