As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON PUSHING POWER BUTTON TO START APU, CHECK FIRE DETECTION WARNING LIGHT CAME ON AND BTC LOW LIGHT ILLUMINATED. APU FIRE VALVE ALSO CYCLED OPEN AND CLOSED SEVERAL TIMES. REMOVED AND REPLACED APU FIRE BOTTLE. OPERATIONAL CHECK OF PRESSURE FOR NEW BOTTLE WORK GOOD AND CHECK OG FIRE DETECTION SYSTEM WORKS GOOD. AIRCRAFT RETURNED TO SERVICE.