As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON FLIGHT 4126 THE CREW REPORTED LAV SMOKE ON EICAS, RAN CHECKLIST, LIGHT ON AFTER FLIGHT ATTENDANT TEST. LIGHT OUT ABOUT 2 MIN LATE ON BASE FOR RETURN LANDING. THE CREW DECLARED AN EMERGENCY AND ELECTED TO RETURN TO ORD. THE AIRCRAFT LANDED WITHOUT INCIDENT. THE AIRCRAFT WAS REMOVED FROM SERVICE. REMOVED AND REPLACED THE AIR CYCLE MACHINE AND HEAT EXCHANGER REF NRWC 685-ORD-01248. THE AIRCRAFT WAS RETURNED TO SERVICE.