Catalyst and Exhaust System

The catalytic converter and exhaust system work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen, carbon dioxide and water vapor. However, it also contains carbon monoxide, oxides of nitrogen, hydrogen, and various unburned hydrocarbons. Carbon monoxide, oxides of nitrogen, and hydrocarbons are major air pollutants, and emission into the atmosphere must be controlled.

The exhaust system consists of an exhaust manifold, Warm Up Three Way Catalytic Converter (WU-TWC), upstream Heated Oxygen Sensor (HO2S), (located just in front of the WU-TWC), Three Way Catalytic Converter (TWC), downstream HO2S (located behind the TWC), a muffler and an exhaust tailpipe.

Typical Heated Oxygen Sensor:

Heated Oxygen Sensors
This vehicle is equipped with both upstream and downstream Heated Oxygen Sensors (HO2S), as OBD II legislation mandates. The Upstream Heated Oxygen Sensor (HO2S 11) is used to control engine fuel flow. The sensor detects the concentration of oxygen in the exhaust gas and sends a signal to the Powertrain Control Module (PCM). In a rich condition, the HO2S will output a voltage of 0.6 volts or greater, and in a lean condition a voltage of 0.4 volts or less will be output.

The downstream Heated Oxygen Sensor (HO2S 12) is used primarily for monitoring the efficiency of the catalytic converter system. The HO2S 12 works in a similar way to the HO2S 11 but only indicates to the PCM that the catalyst is functioning properly. In the event that the HO2S 11 fails, fuel control will be switched over to the HO2S 12 and the Malfunction Indicator Lamp (MIL) will be illuminated. This allows the vehicle to operate with fewer emissions, even with a sensor failure.

Three Way Catalytic Converter (TWC):

Three Way Catalytic Converter (TWC) System
The engine exhaust consists mainly of Nitrogen (N2); however, it also contains Carbon Monoxide (CO), Carbon Dioxide (CO2), Water Vapor (H2O), Oxygen (O2), Nitrogen Oxides (NOx), and Hydrogen (H2) as well as various unburned Hydrocarbons (HC). Three of these exhaust components - CO, NOx, and HC - are major air pollutants, so their emission to the atmosphere must be controlled.

The Three Way Catalytic Converter (TWC), mounted in the engine exhaust system, works as a gas reactor to convert and reduce the pollutant levels to within legally prescribed limits.

The TWC reduces the air pollutants HC, CO, and NOx. The TWC removes these pollutants from the exhaust gases by means of a chemical reaction. The remaining gases are transferred to the muffler.

The catalyst metals are thinly coated onto and supported by a honeycomb shaped high temperature ceramic mounted inside the converter shell. The result is a highly effective converter design having minimum restriction to exhaust gas flow and good durability.





Warm Up Three Way Catalytic Converter
The Warm Up Three Way Catalytic Converter (WU-TWC) is designed to reduce HC, CO, and NOx during warm up conditions.

The WU-TWC is mounted ahead of the TWC, causing it to warm up faster upon vehicle start up. This results in less air pollutants escaping during warm up.