Defects in varistors are detected by pulsing a high current through the varistors and shortly thereafter mapping the temperature distribution of a main varistor surface by heat sensitive equipment. The presence of a localized hot spot sufficiently higher in temperature than a reference temperature level in the varistor indicates the presence of a defect. The disclosed technique is especially adapted for detecting defects that manifest themselves as incipient hot spots but which quickly disappear from observation because their heat rapidly disperses into surrounding varistor material.