What was found
AIRCRAFT WAS AT GATE PRIOR TO BOARDING, THERE WAS AN ONBOARD FIRE IN LAV 3L. PILOT AND FIRST OFFICER HAD ALL OTHER CREW MEMBERS EVACUATE AIRCRAFT. 2 HALON FIRE BOTTLES WERE USED TO EXTINGUISH FIRE. FIRE AND RESCUE WAS CALLED WHEN SMOKE/FIRE WAS FIRST NOTICED. MAINTENANCE REMOVED AND REPLACED FILL SENSOR AND 2 CIRCUIT BREAKERS. OPERATION AND LEAK CHECK GOOD, OK FOR SERVICE JOB COMPLETE.