Chapter 10 Emission Control and Exhaust System Fundamentals 199 Copyright by Goodheart-Willcox Co., Inc. into the engine and burned. Figure 10-30 shows a cutaway view of a catalytic converter with a soot trap. OBD II Catalyst Monitor All vehicles equipped with OBD II use a catalyst moni- tor to ensure that the catalytic converter is operating prop- erly. The monitor also checks for excessively low or high oxygen levels, which could indicate a potential driveability condition. The catalyst monitor uses two or more heated oxygen sensors, Figure 10-31. One or more sensors are mounted in the exhaust manifolds or exhaust pipe upstream from the catalytic converter. A single heated O 2 sensor is mounted downstream from the converter, usually just past the con- verter’s outlet. The catalyst monitor measures the converter’s ability to store and release oxygen. A converter that is operating properly has the ability to store a large quantity of oxygen. When the catalyst begins to fail, its ability to store oxygen decreases. This is sensed by the catalyst monitor. The wave- form can be monitored to show how the monitor works, Figure 10-32. Selective Catalytic Reduction Systems Selective catalytic reduction systems, or SCR systems, are used on vehicles with diesel engines to lower NO x emis- sions. These systems inject a solution of urea and demin- eralized water called diesel exhaust fluid, or DEF, into the exhaust stream. A special catalytic converter downstream of the DEF injection nozzle coverts the DEF to ammonia and carbon dioxide. The ammonia reacts with the NO x , convert- ing it to nitrogen and water. The diesel exhaust fluid tank will have a blue filler cap. Never add anything but approved diesel exhaust fluid to this tank. Diesel Particulate Filter A particulate filter, sometimes called a soot trap, is installed in a diesel exhaust system to collect excess solid particles ahead of the selective catalytic reduction (SCR) system. The particulate trap is a series of baffles made of Figure 10-30. This is a cutaway of a catalytic converter for a die- sel engine. Notice the soot trap. The trap removes soot particles before they reach the catalyst. (Daimler) Soot trap Figure 10-31. Most vehicles have multiple heated oxygen sensors. The heated oxygen sensors allow the computer system to quickly enter closed loop. An oxygen sensor is mounted downstream of the converter to monitor catalyst operation. This exhaust system is used on a diesel engine and also contains two NO X sensors, located after the NO X storage-type catalytic converters. (Daimler) Bank 1, Sensor 1 Bank 2, Sensor 1 Bank 2, Sensor 2 Bank 1, Sensor 2 Temperature sensor NOX storage-type catalytic converters NOX sensors Catalytic converter Catalytic converter