What was found
FLT 4531 - ON FINAL APPROACH INTO ATL, SHORTLY AFTER APU START UP, THE 'SMOKE TOILET' MESSAGE ILLUMINATED. THE CAPTAIN IMMEDIATELY SHUT DOWN THE APU AND TRANSFERRED THE BLEEDS BACK TO THE ENGINE. THE MESSAGE EXTINGUISHED. THE FLIGHT ATTENDANT REPORTED A SLIGHT BUILD UP OF SMOKY HAZE IN THE CABIN THAT QUICKLY DISSIPATED ON LANDING. AIRCRAFT LANDED SAFELY IN ATL. MAINTENANCE REMOVED AND REPLACED THE LEFT COALESCER SOCK AND DUCT TEMPERATURE SENSOR. OPERATIONAL CHECKS NORMAL. AIRCRAFT RETURNED TO SERVICE. (M)