As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX FLIGHT NR 2409: DURING START AND ATTTEMPT TO START APU. THERE WAS A LOUD BANG ALONG WITH A FAIL MESSAGE ON THE POWER SELECTOR SWITCH. MAINTENANCE DETERMINED LOUD 'BANG' WAS ASSOCIATED WITH A FAILURE OF THE APU AND APU ENCLOSURE. REMOVED AND REPLACED TAIL CONE ASSEMBLY, APU ASSY. AND APU DOORS. ALL CHECKS WERE GOOD WITH NO DEFECTS NOTED. AIRCRAFT RETURNED TO SERVICE