As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON ENTERING AIRCRAFT DURING THE BOARDING PROCESS A FLIGHT ATTENDANT AND THE FIRST OFFICER REPORTED DIRTY SOCK ODOR. AIRCRAFT GROUNDED. MAINTENANCE DETERMINED THAT THE CABIN ODOR WAS FROM THE APU BLEED AIR SYSTEM AND SUSPECT THE APU OIL COOLER IS THE SOURCE. CLEANED APU PLENUM AND COMPARTMENT PER JOB CARD 26L00021002, RAN APU NO LEAKS NOTED. PERFORMED DECONTAMINATION OF ENVIRONMENTAL CONTROL SYSTEM PER JOB CARD 26L00021004.