Abstract: A laser welder fault detector system, including a microphone and an associated electronic circuit operatively connected to a solenoid actuated ink jet, is provided to detect the formation of defective welds by an intermittently energized laser and to spray ink on sheets of metal being welded to mark the location of the defective welds. Defective welds are detected by acoustic signals generated during the welding process and picked up by the microphone, the amplitude of the signals, when defective welds are being formed, being relatively low compared to those generated when normal welds are being produced. The electronic components of the system compare the acoustic signals generated during the welding process with a reference signal characteristic of the formation of a normal weld.