As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(LOG 4759244) AFTER STARTING ENGINE 2 STRONG GASSY ODOR FILLED COCKPIT AND CABIN. PERFORMED PACK BURN OUT PROCEDURE PER B737 AMM 21-00 AND REPLACED RECIRC FILTERS PER B737 AMM 21-25. RAN PACKS WITH APU BLEED AND BOTH ENG BLEEDS WITH NO ODOR PRESENT THROUGHOUT A/C.