The factory DEF heater on certain vehicles often fails from fluid intrusion. The Diesel Exhaust Fluid (DEF) Heater, No. 904-642, from Dorman Products is a Dorman OE FIX solution which modifies the OEM design to address this failure point and increase reliability by delivering additional protection from fluid intrusion. Applicable to Freightliner 2017-14, Mercedes Sprinter Van 2018-14. Thoroughly tested, each design undergoes cyclical heater element fatigue testing.