As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE FLIGHT CREW REPORTED THE APU BOTTLE LO CAUTION MESSAGE ILLUMINATED, THE MAINTENANCE DISCREPANCY WAS DEFERRED. THE MAINTENANCE TEAM COULD NOT DETERMINE THE CAUSE OF THE LOW BOTTLE PRESSURE. THE APU FIRE BOTTLE WAS REMOVED AND AN OVERHAULED UNIT WAS INSTALLED. THE AIRCRAFT WAS RETURNED TO SERVICE. DEFECTIVE BOTTLE HAD BEEN IN SERVICE 4 OF 5 YEARS