(Mack/Volvo) If drivers report a major DEF leak or major DEF usage, have the driver check the DEF cap, located behind the DEF fill, under the truck, easily accessible from the passenger side fairing. If the DEF cap is in fact leaking, drivers can use the JBWeld Method to glue the top of the cap. This must dry for at least 6 hours. If the cap cannot be glued, you can attempt to locate parts at a nearby volvo or mack dealer. DEF CAP PART # 21376707 DEF FILTER PART # 23381562