As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED. WHEN EXTERNAL AIR WAS REMOVED AND APU BLEED AND PACK 1+2 COOLING A/C AT GATE, A STRONG DIRTY SOX ODOR WAS OVERWHELMING. MAINTENANCE DETERMINED THAT THE SOURCE OF THE ODOR WAS THE APU AND WAS PLACED ON MEL. MAINTENANCE FOUND EVIDENCE OF OIL LEAKING FROM THE APU OIL CAP (FILLER PLUG ASSEMBLY) AND REPLACED IT ALONG WITH THE ASSOCIATED PACKING. RESIDUAL OIL WAS CLEAN FROM THE APU AND A SATISFACTORY LEAK AND OPERATIONAL CHECK WAS ACCOMPLISHED WITH NO ODORS NOTED.