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Home >> Audi >> 2007 >> Q7 4.2 >> Repair and Diagnosis >> Engine Performance >> Engine Control Systems >> 4.2L - Generic Scan Tool - Engine Code: Bar (General Information) >> St Generic Scan Tool >> Description And Operation >> SCR NOX Catalyst System >> Notes
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SCR NOX Catalyst System: Notes

In order to convert harmful exhaust gas components (nitrogen oxides) to environmentally compliant components (nitrogen and water) by means of an SCR catalyst, a dosing valve injects the required amount of reductant into the exhaust system upstream of the SCR catalyst according to the actual demand. The SCR system is controlled and monitored by the SCR control module, which communicates with the ECM via CAN Bus. The SCR control module sends its diagnostic information to the ECM by means of CAN messages. The faults are stored in the fault memory of the ECM. Due to the additional SCR catalyst located downstream of the oxidation catalyst and the diesel particulate filter, nitrogen oxide emissions can be nearly eliminated. A urea solution is used as a reducing agent (the reducing agent is also referred to as Diesel Exhaust Fluid), which is injected into the exhaust system in small quantities. The amount of injected reductant depends on the temperature upstream of the SCR catalyst, the exhaust gas flow rate, and the raw NOx emissions. The target of the dosing strategy is to provide the SCR catalyst with a sufficient amount of reductant for all driving conditions.

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