As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PREFLIGHT ACTVITIES STARTED APU. AFTER APU START AND WAITING FOR ELECTRICAL SWITCH OVER, GOT A LEVEL 3 CARGO FIRE LWR1 ALERT. FOLLOWED QRH AND DISCHARGED FIRE BOTTLE (FLASHING DISCHARGE SWITCH). REMOVED AND INSTALLED LOWER CARGO COMPTMENT FIRE EXTINGUISHER IAW MM 26-22-01-4 AND INSTALLED LOWER CARGO COMPARTMENT FIRE DISCHARGE CARTRIDGE IAW 26-22-04-4 AND TEST LOWER CARGO FIREX DISCHARGE CURCUIT IAW MM 26-22-01-5. OPS CHECK NORMAL. OK FOR SERVICE. PREMATURE REMOVAL/INSTALLATION.