As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON FLIGHT 3989 DURING THE TAXI THE FLIGHT ATTENDANT REPORTED A STRONG SULFUR SMELL WITH A THICK HAZE IN THE CABIN. THE CREW ALSO RECEIVED A LEFT PACK AUTO FAIL CAUTION MESSAGE. THE AIRCRAFT WAS REMOVED FROM SERVICE. REMOVED AND REPLACED THE LEFT AIR CYCLE MACHINE PACK IAW CRJ-700 AMM 21-51-08. THE OPERATIONAL TEST AND LEAK TEST WERE GOOD. THE AIRCRAFT WAS RETURNED TO SERVICE.