As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: UNPLEASANT ODOR OF UNKNOWING ORIGIN PRESENT AT LOW ALTITUDE AND IDLE THRUST WITH CABIN DIFFERENTIAL PRESSURE AT ZERO. SUSPECT AIR CONDITIONING AS SOURCE. NO SMOKE OR HAZE PRESENT. ODOR WAS MUSTY / DIRTY SOCK SMELL. MAINTENANCE ISOLATED THE SOURCE OF THE ODOR TO THE APU AFTER FINDING EVIDENCE OF OIL IN THE APU INLET, LOAD COMPRESSOR. **SUPPLEMENTAL REPORT WILL BE SUBMITTED** (OPEN)