As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT 5005, RETURNED TO GATE FOR ODD SMELL FROM PAX. AIRFLOW SMELL GOT WORSE ONCE ENGINE START AND APU TURNED OFF. FLIGHT ATTENDANTS COMPLAINED OFF A HEADACHE. CORRECTIVE ACTION: REPLACED APU, CLEANED APU COMPARTMENT AND REPLACED RECIRCULATION FILTERS. (LP 5254368)