As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT WAS GROUNDED. STRONG ODOR IN CABIN FLIGHT DECK UPON ARRIVAL AT AIRCRAFT. SMELLS OF MUSTY SOCKS. COMPLIED WITH JOB CARD 26L00021001 AND FOUND APU AS SOURCE OF ODOR. RAN PACKS WITH ENGINE SOURCE WITH NO ODOR DETECTED. REPLACED APU PER CARD 26L31049003. ALL OPERATIONS CHECK GOOD.