As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS ON APU START UP APU EGT WENT TO 900 DEGREE C ACCOMPLISHED APU EMERGENCY SHUTDOWN. FIRE VISIBLE IN COMPARTMENT HAD TO USE HALON TO BLOW OUT FIRE INTERNAL IN APU. AN APU CHANGE HAS BEEN DECLARED. REMOVED AND REPLACED APU IAW JOB CARD 26L31049003. OPS CHECK GOOD.