As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE AT GATE WITH APU RUNNING A BURNING ODOR STARTED IN THE AFT CABIN AND MIGRATED TO THE FRONT. CUSTOMERS EXITED THE ACFT NORMALLY. MAINTENANCE TROUBLESHOT AND INSPECTED APU AND APU INLET. NO DEFECTS NOTED. PERFORMED DUCT BURNOUT AND REPLACED COALESCER BAGS.