As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EWR - CREW REPORTED STRONG BURNING SMELL THROUGHOUT CABIN. EMERGENCY DECLARED, RETURNED EWR WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. PERFORMED MCM CARD 21-96-01 CABON ODOR TROUBLESHOOTING. REPLACED LEFT PILOT VALVE AND LEFT PACK PRESS REGULATING VALVE. SYSTEM GROUND CHECK NORMAL OPERATION.