Three failed modules were found during routine inspection of industrial equipment after several years of service. We investigated failure causes of them. As a result of the investigation, we show that there are different failure modes for each type of variable resistors (cermet type, carbon composition type, wire wound type). (1)Cermet type: The wiper was unstable because the wiperfixing was weak already during manufacturing. (2)Carbon composition type: The R12 (resistance between #1 and #2 terminals) was unstable because of extraneous materials in manufacturing or zinc whiskers. (3)Wire wound type: The R13 (resistance between #1 and #3 terminals) was broken by the scratch in manufacturing.