As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN APU BLEED VALVE WAS TURNED ON WE SMELLED SIGNIFICANT DIRTY GYM SOCK SMELL. THIS HAPPENED 3 TIMES DURING AIR SOURCE SWITCH OVER. SUSPECT APU SOURCE OF CONTAMINATION VERIFIED. R&R APU PER JOB CARD 4900-9901E AMM 49-11-01 PNEUMATIC DUCT STILL NEEDS CLEANING(681367) COMPLETED W/C 4900-9901E C/W NO GO ODI PREVENTION PROGRAM 2172. C/W AMM 21 00 01 002 001. NO ODOR PRESENT.