What was found
DURING CLIMB OUT, APU FIRE LIGHT ILLUMINATED, FOLLOWED QUICK REFERENCE HANDBOOK & DISCHARGED APU FIRE EXTINGUISHER BOTTLE. TURNED BACK & LANDED WITHOUT EVENT. MAINTENANCE INSPECTED APU AREA AND FOUND NO DAMAGE, PLACED APU ON MEL 49-1, COND. 1 "APU INOP". WHEN AIRCRAFT ARRIVED IN PHX MAINTENANCE PERFORMED AN IN DEPTH INSPECTION OF THE APU ASSEMBLY, TAILPIPE, AND SURROUNDING AREA, NO EVIDENCE OF FIRE OR DAMAGE WAS NOTED. REPLACED APU FIRE EXTINGUISHER BOTTLE AND YELLOW DISCHARGE DISK IAW BMM 26-22-11. DURING C-16 CHECK MAINTENANCE REMOVED AND REPLACED THE APU.