As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
TURNED ON LEFT ENGINE NACELLE HEAT, ENGINE FIREWALL DETECTION ANNUNCIATOR ILLUMINATED. FOLLOWED CHECKLIST TURNED OFF LEFT ENGINE NACELLE HEAT AND ANNUNCIATOR EXTINGUISHED. LANDING WAS UNEVENTFUL. MAINTENANCE TROUBLESHOT THE DISCREPANCY TO A UNSECURE LEFT ENGINE P3 BLEED SUPPLY LINE. LINE WAS SECURED AND A SUCCESSFUL SYSTEM OPERATIONAL CHECK WAS PERFORMED. AIRCRAFT WAS RETURNED TO SERVICE.