Failure of Auto tensioner was observed at engine of LCV segment vehicles. In this study both vehicle samples were analyzed to have a detailed investigation on the issue. In common, these failures can be caused due to assembly issues, adjacent parts incorrect mechanical parameters (belt tension, Arm movement, Angular vibration) and also by improper material selection etc. Issue was identified in pivot bush which is one of the sub assembly part in Auto tensioner. Where the bush material is with glass as a fillers in base polymer material of Nylon 66 matrix against the material specification on Nylon 66 with PTFE, MoS2 and Aramid as fillers. As glass is abrasive in nature eroded the mating part, hence caused the Auto tensioner system failure. This study revealed the failure is due to improper material composition of the pivot bush (sub assembly part) caused by raw material mix up at tier 2 supplier’s premises Keywords: IC Engine, Belt Auto Tensioner, Field Failure, Tear Down Analysis, SEM Analysis, ASTM D5630