A cold air intake is an aftermarket engine modification that replaces a vehicle's stock air intake system. A cold air intake typically draws in cooler air from outside the engine bay, which can result in increased power and improved fuel efficiency. Many people believe that cold air intakes are especially beneficial for diesel trucks, as the cooler air can help to increase combustion and improve engine performance. However, it's important to note that there is no definitive proof that cold air intakes provide any significant benefits for diesel trucks. In fact, some experts believe that cold air intakes can actually be detrimental to a diesel engine's performance, as they can allow water and other contaminants to enter the engine.