As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FIRE WARNING SOUNDED WHILE AT GATE RUNNING APU FOR 5 MIN WITH GEN AND PACKS ON. CREW SHUTDOWN APU AND FIRED NR 1 EXTINGUISHER. FOUND FLEX COUPLING AT SURGE CONTROL VALVE BLOWN. REPLACED NR 1 FIRE EXTINGUISHER AND PUT APU ON MCO 49-00-01A.