On January 11, 2002, at 0045 mountain standard time, a Boeing 757-222, N551UA, operating as United Airlines flight 23, sustained minor damage when, during cruise flight, a fire occurred in the aft lavatory. The fire was extinguished, an emergency declared, and an uneventful landing was made at the Salt Lake City International Airport (SLC), Salt Lake City, Utah. Night visual meteorological conditions prevailed during the time of the incident. The scheduled domestic passenger flight was being operated on an instrument flight rules plan under the provisions of Title 14 CFR Part 121. The captain, first officer, 5 flight attendants, and 133 passengers reported no injuries. The flight originated at San Francisco, California, at 2228, and was en route to Boston, Massachusetts. The company flight crew reported that flight 23 was at a cruise altitude of 37,000 feet mean sea level and approximately 95 nautical miles south of SLC, when a flight attendant called from the aft cabin and stated there was a fire in the left aft lavatory, and another flight attendant was fighting the active flames with a Halon extinguisher. The captain said he declared an emergency and requested priority handling for landing at SLC. The captain reported performing a category II approach and autoland at maximum landing weight. A flight attendant stated that a passenger on exiting the lavatory reported smelling smoke and that the side of the toilet was "bubbling." Another flight attendant entered the lavatory to inspect. On opening the door to the heater, the flight attendant stated that flames were "shooting up" around the base of the heater. The flight attendant was handed a Halon extinguisher. She immediately discharged its contents to the area extinguishing the flames. The flight attendants continued to monitor the lavatory until the airplane landed. An examination of the airplane revealed the lavatory toilet water level sensor was charred and melted. The inside aft wall of the toilet shroud, in the area over the sensor, was charred and melted. Vent tubes in the area were also charred and melted. The 5 ampere (amp) circuit breaker to the sensor was examined and found tripped. No other anomalies were found. The water level sensor, shroud, vent assembly, and circuit breaker were retained for further examination. The lavatory sensor was examined at Rosemount Aerospace, Burnsville, Minnesota, on March 6, 2002. The examination showed that most of the top cap of the unit was charred, melted, and consumed by fire. A portion of the outer wall of the lavatory sensor housing was also charred, melted and consumed by fire. The remainder of the cap was removed exposing the unit's silicon potting material. The potting material showed a crack spanning the diameter of the sensor located above the top edge of the internal circuit board. There was charring and melting of the potting material along the crack. X-rays of the sensor showed the top left corner of the circuit board melted. This included solder attachments for four wires, two of which were a 115-volt AC input wire and 28-volt DC ground wire. The x-ray examination of the unit also showed portions of the wires melted and consumed. The circuit breaker was examined and tested at United Airlines, San Francisco, California, on March 6, 2002. The examination revealed the circuit breaker spring as "weak." The circuit breaker was tested at 6.9 amperes, 138 percent of its normal current load for 1 hour as per manufacturer's specifications. According to the specifications, the circuit breaker is required to trip within the 1 hour time period. After 1 hour, the circuit breaker had not tripped. The current was increased to 7.25 amps and maintained for 10 minutes. The circuit breaker still did not trip. The current was then increased to 7.5 amps and maintained for 4 minutes. The circuit breaker still did not trip. The current was increased to 7.7 amps. After 1 minute, the circuit breaker tripped. A new circuit breaker was then tested at 6.9 amps. It tripped within 2 minutes. The lavatory sensor was examined a second time at Rosemount Aerospace, Burnsville, Minnesota, on April 22, 2003. During the examination, the sensor housing was opened and the silicon potting material was removed from around the circuit board. The top left corner of the circuit board was charred and consumed. The left side of the circuit relay, adjacent to the charred area, was melted and had melted solder on its exterior. The manufacturer determined that solder used on the circuit board was "probably 63/67 with a melting point of 361 degrees Fahrenheit (F). The left hexagon-shaped screw, one of the two screws that mount the relay to the circuit board, was melted. The manufacturer stated that the screws were probably made from "304 stainless steel" and determined the melting point of the metal to be between 2,550 and 2,650 degrees F. The investigation team determined that for these temperatures to occur, one of the wires in the area of the left screw would have had to arc. Removal of additional silicon potting material showed that portions of three jumper wires were melted and consumed. The manufacturer stated that the Teflon sheathing that surrounded the wires had a melting temperature of 500 degrees F. The lavatory sensor was manufactured on June 15, 2002. It was installed at Indianapolis, Indiana, during the airplane's heavy maintenance visit on October 28, 2002. The total airframe time at the heavy maintenance visit was 40,792 hours. The airplane was put back in service on November 17, 2002. The airplane was at 41,361 hours when the incident occurred. An examination of a new lavatory sensor circuit board showed the 115-volt AC input wire and the 28-volt DC ground wire were soldered to the board next to each other, approximately 1/64th-inch apart. Parties to the investigation were the Federal Aviation Administration, Salt Lake City, Utah, United Airlines, the Boeing Aircraft Company, and Rosemount Aerospace.