This manual covers the replacement procedure for the DD 13/15/16 HT2 Heater Return Line, identified by service bulletin number 83-131. It is applicable to a range of Freightliner vehicle models, including the FLA COE, FLB COE, FLD Conventional, Business Class, FLC 112 Conventional, Century Class Conventional, Argosy COE, Cargo, Columbia, Coronado, Business Class M2, and Cascadia. These vehicles are known for their robust engineering and reliability, designed for various commercial applications. The purpose of this manual is to provide detailed, step-by-step instructions for the correct and safe replacement of the specified heater return line. It is intended for use by qualified technicians and service personnel to ensure proper maintenance and repair of the vehicle's heating system. Adhering to the procedures outlined herein will help maintain the operational integrity and longevity of your Freightliner vehicle.