1517737 NEW PRIMARY ENGINE AIR FILTER Fits CaterpillarOne New Aftermarket Replacement Primary Engine Air Filter Fits Caterpillar (Fits CAT) Construction Equipment and Generator Sets This aftermarket replacement primary engine air filter (filter element assembly) protects Fits Caterpillar (Fits CAT) engines from damaging airborne contaminants. The primary filter is the main filtration stage in the engine air cleaner assembly, capturing dust and debris before they can reach internal engine components.