The emission system controls the vehicle’s emissions, exhaust and pollutantsn using an array of sensors, computerized engine controls and the exhaust components. Emission systems substantially reduce harmful gases such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx), and prevent harmful gasoline vapors from escaping at the fuel tank. Your car’s emission system keeps the engine running cleanly and efficiently in all types of operating conditions. Keeping it in proper working condition can save you money and protect the environment. Fixing a faulty oxygen sensor can improve gas mileage by 40%!