As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING DE-ICING AT GATE, EXPERIENCED AN APU FIRE WARNING IN THE COCKPIT. THE APU WAS SHUT DOWN AND THE FIRE BOTTLE WAS DISCHARGED. EVIDENCE OF FIRE WAS FOUND, THE FIRE BOTTLE WAS REPLACED AND THE APU WAS PLACED ON MEL. THE APU WAS LATER REPLACED.