With Direct Injection Engines, when you accelerate your car it releases gas into the combustion chamber of the vehicle. Spark Plugs then ignite the gasoline causing the car to move. Carbon is a byproduct of this process, and the carbon builds up and coats the inside of your engine. Carbon can cause the car to lose power, misfire, and sometimes cause your check engine light to turn on. This happens only with Direct Injection Engines because the injectors are placed directly in the combustion chamber. With a normal Port Injection Engine, the injectors are placed before the valves and are cleaned as the engine runs.