As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
STL TO IAH - AFTER TAKEOFF FROM STL FLIGHT CREW RECEIVED A LAV SMOKE EICAS MESSAGE. AT THE SAME TIME THE FLIGHT ATTENDANT CALLED THE COCKPIT TO REPORT SMOKE IN THE CABIN COMING FROM THE LAV TO ABOUT MID CABIN. THE FLIGHT CREW ALSO DETECTED A FAINT ELECTRICAL BURNING SMELL IN THE COCKPIT. THE FLIGHT CREW FOLLOWED THE QRH AND MADE AN IMMEDIATE RETURN TO FIELD. THE AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED THE AIRCRAFT AND RAN ENGINES AND TRACED THE SOURCE OF THE SMOKE TO THE NR 2 ENGINE. THE NR 2 ENGINE WAS REPLACED AND THE SMOKE DID NOT RETURN.