As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH APU RUNNING ON GROUND AT GATE AND APU BLEED ON, AN ODOR RESEMBLING OIL OR HYDRAULICS WAS NOTED BY SEVERAL CREW MEMBERS. MAINTENANCE REMOVED AND REPLACED SERVO VALVE AND BASE PLATE OF RIGHT YAW DAMPER PER IAW AMM 27-26-51-400-002-A. LEAK AND OPS CHECK SATISFACTORY. ACCOMPLISHED J/C 26L00021001 AND J/C 26L00021004.