What was found
THE APU FIRE LIGHT ILLUMINATED DURING CLIMBOUT OF DEN. THE FIRE HANDLE WAS PULLED AND THE FIRE BOTTLE WAS DISCHARGED. THE LIGHT WENT OUT, BUT ILLUMINATED AGAIN WITH THE FIRE BELL SOUNDED. THE AIRCRAFT WAS RETURNED TO DEN WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED THE APU COMPARTMENT AND APU, AND FOUND NO EVIDENCE OF FIRE. THE FORWARD SIDE OF BULKHEAD IN TAIL COMPARTMENT WAS ALSO INSPECTED WITH NO DISCREPANCIES FOUND. THE ENGINE FIRE CONTROL MODULE WAS REMOVED AND REPLACED. THE APU LOWER FIRE DETECTION LOOP WAS ALSO REPLACED DUE TO A DAMAGED LOOP. THE APU FIRE BOTTLE WAS REMOVED AND REPLACED IAW MM 26-22-11, PAGE 401-406. AN OPERATIONAL CHECK OF THE APU FIRE PROTECTION SYSTEM WAS GOOD.