Catalyst and Exhaust Systems: Overview

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.

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 vapor (H ₂ O). However, it also contains carbon monoxide (CO), oxides of nitrogen (NO _x ), hydrogen (H), and various unburned hydrocarbons (HCs). CO, NO _x , and HCs are major air pollutants, 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 will be used between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the on-board diagnostic (OBD) system strategy in the PCM. For information on the OBD catalyst monitor, refer to the description for the CATALYST EFFICIENCY MONITOR .

The number of HO2S used in the exhaust stream and the location of these sensors depend on the vehicle emission certification level (i.e. LEV, ULEV, PZEV). On most vehicles only 2 HO2S are used in an exhaust stream. The front sensors (HO2S11/HO2S21) before the catalyst will be used for primary fuel control while the ones after the catalyst (HO2S12/HO2S22) will be used to monitor catalyst efficiency. However, some partial zero emission vehicles (PZEV) will use 3 HO2S sensors for each engine bank. The stream 1 sensors (HO2S11/HO2S21) located before the catalyst are used for primary fuel control, the stream 2 sensors (HO2S12/HO2S22) are used to monitor the light-off catalyst, and the stream 3 sensors (HO2S13/HO2S23) located after the catalyst are used for long term fuel trim control to optimize catalyst efficiency (fore aft oxygen sensor control). Current PZEV vehicles use only a 4-cylinder engine, so only the Bank 1 HO2S will be used.

Fig 1: Identifying V- Engines HO2S Stream Location And Naming Conventions

Courtesy of FORD MOTOR CO.

Fig 2: Identifying In-Line Engines HO2S Stream Location And Naming Conventions

Courtesy of FORD MOTOR CO.