The Mercedes-Benz C-Class, including the C300, uses a dual air filter design to keep the engine air intake as clean as possible to maximise engine performance. The filters mount underneath the large engine cover and are accessible by removing the cover from the engine compartment. You can buy replacement filters from an auto parts retailer. Change the filters at least every 30,000 miles, or replace them more often if you drive in dusty conditions.
Open the bonnet of the car. Grasp the sides of the smaller engine cover in the front of the engine compartment and pull up to disengage the clips. Work around all four edges to release the cover, then set it aside.
Disconnect the air hose running to the rear engine cover. Pull up on the cover, working around all four edges. Turn the cover upside down to access the two engine air filters.
Pull the air filters out of the mounting area. Slide the replacement filters into place. Push the rear engine cover into place until the clips lock. Align the front engine cover and press it into place.