What was found
PASSENGERS HAD TO DEPLANE AFTER BOARDING BUT BEFORE PUSHBACK DUE TO SMOKE IN THE AFT CABIN. MAINTENANCE REPORTED THAT THERE WAS ALSO A BURNING SMELL. ENGINES WERE OFF, APU BLEEDS ONLY. NO INJURIES. FLIGHT IS BEING SWAPPED TO A DIFFERENT AIRCRAFT. BURNING SMELL AND HAZE IN BACK OF A/C APU BLEEDS ON, ENGINES OFF / FOUND AFT LAV EXHAUST FAN TO BE THE SOURCE OF THE BURNING SMELL. THE EXHAUST FAN IS LOCATED DIRECTLY UNDER THE FLOOR FROM THE SMELL SOURCE. FOUND THE CB TO BE OPEN AND WILL NOT RESET. PART NOT IN STOCK FOR REPLACEMENT. SEE TRFKE007DKT5 FOR DEFERRAL. DEFERRED AFT LAV EXHAUST FAN PER MEL 21-23-02-2 CAT C. M FUNCTION CW BY INSTALLING PLACARDS AND OPENED AND COLLARED CB B8 ON CBP1 FOR DEACTIVATION.