Idle speed and mixture adjustments are not a part of the manufacturer's maintenance schedule. On gasoline models, the idle speed and air/fuel mixture are controlled by the ECM, making routine adjustments unnecessary. Diesels (with the exception of the computer-controlled AHU TDI engine) use extremely precise mechanical components to regulate idle speed and air/fuel mixture, making routine adjustment unnecessary. If the engine is not in proper running order, a faulty sensor or other component is usually the cause. Refer to for more information.