As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT RETURNED TO THE GATE DUE TO A STRONG SULFUR SMELL IN THE CABIN. MAINTENANCE FOUND A DAMAGED IMPELLER ON THE RIGHT RECIRCULATION FAN RESULTING FROM CHECK VALVE FLAPPER DOOR FAILURE POSITIONED AT THE LOWER PLENUM FOR RECIRCULATION AIRFILTER. THE RIGHT RECIRCULATION FAN AND CHECK VALVE WERE REPLACED PER AMM. SYSTEM OPERATION IS NORMAL.