As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: VERY STRONG HYDRAULIC OIL ODOR COMING THRU THE CONDITION AIR. ONLY SMELLED IN THE FLIGHT DECK. REPLACED THE LEFT AND RIGHT HAND RECIRCULATION AIR FILTERS AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK PER AMM 21-21-41 AND TASK CARD 26L00021001