Replace your old and dirty air filter with a Drop-In Air Filter for your 1998.5-2002 Dodge 5.9L Diesel. Replace the factory air filter on your vehicle with a quality drop-in air filter from S&B Filters, AFE, K&N Engineering, and others at XDP. An air filter is a crucial component for your engine and its purpose is to keep harmful airborne dust and debris out of your engine. A drop-in air filter is named after the simple installation procedure as it simply drops into the factory airbox in most applications. In most cases an oe-style drop-in air filter will be made with paper filter media, where an aftermarket air filter typically features a synthetic media, or a cotton gauze design that may be dry or oiled.