A fender liner (sometimes called a “front fender liner”) is a vehicle body component that is designed to shield other components from moisture, dirt, and debris.
The outer body panels at the front of your vehicle along each side of the hood and directly ahead of the front doors are called fenders. A fender is made of the same material as the rest of the vehicle body, typically stamped sheet metal. Most fenders form a hollow shell that helps to lend shape to the front of a vehicle.
Because the fender is usually hollow, any contaminants - water, snow, salt, dirt, and debris - that can be kicked up by the front tires would become trapped on the inside. The engine compartment would also be subject to the elements as well. That is to say, if a fender liner was not in place.
The fender liner is a thin, dome-shaped plastic (sometimes metal) panel attached to the inner edges of the fender and to the body structure. The liner forms a semi-hollow well that the wheel can travel in along with the suspension. Held in place with clips and screws, the fender liner provides a dirt and moisture proof covering for the inside of the fender and the engine compartment.
The fender liner on many older vehicles, those produced decades ago, were often made of sheet metal. Those liners were difficult to remove and install and were subject to corrosion. A rusted fender liner - or one that was damaged in an impact - would need to be replaced.
Modern fender liners are made of thin, lightweight plastic. Although tricky to replace in some cases, they are far simpler and more versatile than the old metal components. Unfortunately, they are more susceptible to damage from road debris than metal liners. If you run over a foreign object in the road, it is quite possible that the fender liner might become damaged. Same goes if you run over a parking block or other obstruction.
With a plastic liner, you will know you need a new one if its mounting surfaces are torn away from the fender, body structure, or lower part of the front bumper. A strip or flap of plastic hanging out of the fender well or coming into contact with the front tire would indicate that a new fender liner needs to be installed.