As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED APU FIRE MESSAGE, APU BOTTLE LOW AND APU AUTOMATICALLY SHUT DOWN ON TAXI OUT AT DCA. NO VISIBLE SMOKE OR FIRE. MESSAGE EXTINGUISHED. CREW FOLLOWED QRH AND RETURNED TO THE GATE. MAINTENANCE INSPECTED APU COMPARTMENT AND FOUND NO VISUAL SIGNS OF FIRE OR LEAKS. MAINTENANCE REMOVED AND REPLACED APU FIRE BOTTLE, APU FIRE BOTTLE DISCHARGE CARTRIDGE AND FIDEX CONTROL UNIT AND PERFORMED CHECKS USING THE AIRCRAFT MAINTENANCE MANUAL. AIRCRAFT WAS RELEASED FOR SERVICE.