Mud guard?

Under the front bumper of my Kia Rio is a plastic guard that hangs down for some purpose. My brother-in-law thinks its for aerodynamics. All I know is that it has cracked and is hanging down on one side. Does it serve any purpose? Should I just take it off, or should I replace it? (Most of this car seems to be plastic)

It is for aerodynamics, as well as directing airflow through the radiator. Without it, it is possible the engine will run hotter at highway speeds.

What is it actually called?

An air dam. It funnels air to the radiator.

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most manufactures call this a “lower deflector”. I have never heard anyone complain that thier car runs hot without it but if it is still there reattach it.

It’s an air dam and they all fall off eventually, with no obvious detrimental effects to your car. If it were truly important they’d make them out of something more substantial than plastic.