Catalyst – how to correctly diagnose its operation?

Catalytic converters or catalytic converters, because these are the correct names of catalysts currently used in all newly manufactured vehicles, are an important element of the exhaust system. The catalyst’s task is to reduce harmful compounds entering the atmosphere from exhaust gases, primarily carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx).


Emission control regulations have a history of almost 60 years. They were first introduced into applicable US law in 1960. Also in the United States, as one of the first countries (later joined by Japan and Europe), not only the obligation to use catalytic converters in exhaust systems was introduced, but also a uniform on-board diagnostics system. The latter included, above all, monitoring the effectiveness of catalysts, obviously in the limited scope of emission control of harmful exhaust compounds at that time.


Externally, the catalytic converter is a tin can reminiscent of a muffler in the exhaust system. Due to the material used for construction, we distinguish two types of catalysts: ceramic with ceramic block and metal with metal block. The interiors of the blocks contain a large number of channels with a honeycomb structure. They are covered with a layer of precious metals: most often it is platinum, but rhodium and palladium catalysts are also mounted. Precious metals undergo a chemical reaction with toxic components contained in the exhaust gas, resulting in the reduction of the latter.


One of the most commonly used elements in cars with a gasoline engine is the TWC three-way catalyst (from Three Way Catalyst). The name comes from its action, i.e. the simultaneous reduction of nitrogen oxides (NOx) and the oxidation of hydrocarbons (HC) and carbon monoxide (CO). On the other hand, in self-ignition units, only HC and CO oxidation reactors are used. Simultaneous NOx reduction is impossible due to the fact that these engines work on lean mixtures.


Not everyone knows that in vehicles manufactured in the first years of the 21st century, the efficiency of catalytic converters was not controlled by the on-board diagnostic system. Consequently, the owner could move the car with a malfunctioning system without being aware of this fact. The situation changed in January 2005, when the Euro IV regulations came into force. They introduced the obligation to monitor the efficiency of catalytic converter operation by the on-board self-diagnostics system of the vehicle. When the efficiency of the catalytic converter is significantly reduced, the driver is informed of the situation by lighting up the MIL (Malfunction Indicator Light) on the instrument cluster.


Two probes are used for the self-diagnosis system to measure the oxygen content in the exhaust gas. One of the probes is located in front of the catalytic converter (control of some emission reduction systems and engine operation), the other one behind the catalytic converter (control of some emission reduction systems, as well as the catalyst itself). When the catalytic converter is warmed up to operating temperature, the probe – located behind the catalytic converter – should maintain a voltage level of about 0.8V. With a good catalyst, the exhaust gas contains a small amount of free oxygen molecules. This is normal because oxygen is used in the catalytic process to oxidize harmful gases to form harmless gases. However, if there is damage to the catalyst or at least a significant reduction in its efficiency, the probe after the catalyst will indicate a similar amount of oxygen molecules to the probe before the catalyst. Consequently, the probe voltage after the catalyst will oscillate between 0.8 and 0.45V, depending on the proportion of the air-fuel mixture controlled by the electronic control unit. Deviations in operation from the values stored in the control unit’s memory cause the corresponding error code to be saved in the memory and the above-mentioned MIL indicator on the instrument cluster lighting up. TEAM

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