A properly functioning emissions system provides improved gas mileage, better horsepower and helps the environment by producing less harmful exhaust. Emission systems are made up of two key components, the catalytic converter and oxygen sensors.
Closely resembling a muffler in appearance, the catalytic converter is located in the exhaust system and has an outer shell made of stainless steel. The similarity with a muffler ends there as catalytic converters contain a catalyst made from a noble metal such as platinum, palladium or rhodium. A catalyst is defined as anything that induces or accelerates a change. At least one catalytic converter has been used on cars since 1975; today, cars may have two or more depending on the engine configuration and manufacturer.
Purpose: Using its internal catalyst, a catalytic converter's job is to greatly reduce the level of harmful emissions in a car's exhaust. Namely, these are carbon monoxide, hydrocarbons and oxides of nitrogen. All of these emissions are serious health and environmental hazards, plus they contribute to the formation of photochemical smog. A catalytic converter changes these poisonous gases to harmless carbon dioxide, nitrogen, oxygen and water. In a simplistic way, the catalytic converter can almost be thought of as an engine of its own. The converter uses fuel and oxygen to "light off" its internal catalyst which consumes a large portion of the gases flowing through the converter. Although a converter greatly reduces emissions, it does not eliminate them altogether.
Located in the exhaust system at one or more points, an oxygen sensor converts the status of the engine's air/fuel mixture into a corresponding voltage signal. The oxygen sensor is sometimes referred to as a Lambda sensor on some European cars. The most common type of sensor uses a zirconia element to generate voltage. Titania oxygen sensors, on the other hand, use the technology of electrical resistance to produce similar voltage characteristics as zirconia sensors. Regardless of the technology used, changes in the air/fuel mixture result in a voltage change that is monitored by the vehicle's powertrain control module. Many oxygen sensors also have built-in heaters, to warm them to operating temperature more quickly. Prior to 1996, most cars had one oxygen sensor. In 1996, Onboard Diagnostics II (OBDII) became standard technology for all makes and models. OBDII systems use one oxygen sensor on the inlet to the catalytic converter and another one on the outlet. On cars with dual converters, there may be as many as four oxygen sensors.
Purpose: The oxygen sensor reports live information about the engine's air/fuel mixture to the power train control module. This information is used primarily to help calculate fuel delivery to the engine, which changes continuously while it is running. If the engine is running lean, the power train control module will sense this from the oxygen sensor's signal and increase the air/fuel mixture to the engine. Conversely, just the opposite occurs when the engine begins to run rich. On OBDII-equipped vehicles, the sensors are also used to help determine the efficiency of the catalytic converter. The power train control module does this by comparing the signal of the sensor located at the inlet of the catalytic converter with the signal of the sensor located at the outlet of the converter.