Catalyst and Exhaust Systems: Overview

The catalytic converter and exhaust systems work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen (N), carbon dioxide (CO₂ ) and water (H₂ O). However, it also contains carbon monoxide (CO), oxides of nitrogen (NO_x ), hydrogen (H), and various unburned hydrocarbons (HCs). The major air pollutants of CO, NO _x , and HCs, and their emission into the atmosphere must be controlled.

The exhaust system generally consists of an exhaust manifold, front exhaust pipe, front heated oxygen sensor (HO2S), rear exhaust pipe, catalyst HO2S, a muffler, and an exhaust tailpipe. The catalytic converter is typically installed between the front and rear exhaust pipes. On some vehicle applications, more than one catalyst is used between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the on board diagnostic (OBD) system strategy in the powertrain control module (PCM). For information on the OBD catalyst monitor, refer to the description for the CATALYST EFFICIENCY MONITOR .

For most vehicles, only two HO2Ss are used in an exhaust stream. The front sensors (HO2S11/HO2S21) before the catalyst are used for primary fuel control while the ones after the catalyst (HO2S12/HO2S22) are used to monitor catalyst efficiency. However, some partial zero emission vehicles (PZEVs) use three HO2Ss. The stream 1 sensor (HO2S11) located before the catalyst is used for primary fuel control, the stream 2 sensor (HO2S12) is used to monitor the light-off catalyst, and the stream 3 sensor (HO2S13) located after the catalyst is used for long term fuel trim control to optimize catalyst efficiency (fore aft oxygen sensor control).