As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: MUSTY ODOR COMING FROM AIR VENTS APU RUNNING. MAINTENANCE ISOLATED THE SOURCE OF THE ODOR TO THE APU. MAINTENANCE FOUND THE APU HAD BEEN PREVIOUSLY OVER SERVICED RESULTING IN THE APU BEING CONTAMINATED. MAINTENANCE CLEANED THE APU PLENUM, INLET AND LOAD COMPRESSOR PER AMM 49-00-00 AND AMM 49-20-00. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED WITH NO ODORS NOTED.