Due to the independent operation of the aftertreatment system from the engine, the aim of the OBD system (On Board Diagnostic) is to guarantee the powertrain emissions stay within the emissions standards during all the vehicle useful life. In the case of heavy-duty diesel vehicles, that uses the SCR system (Selective Catalytic Reduction) as the aftertreatment technology to meet the stringent emissions levels, the use of a NOx sensor in the exhaust pipe will be indispensable for a reliable monitoring of the emissions.Analyzing the European directives that are legislating the OBD system, the NOx sensor must be monitored by the emissions control system, guaranteeing faults detection in the aftertreatment system that could increase the emissions. According to the annex 4 of the directive 2005/78/EC item 18.104.22.168 (requirements for the OBD system Stage 2), it is needed to monitor the catalyst efficiency as an isolated component of the system.In case of efficiency reduction of the aftertreatment system, due to the low quality of the SCR reagent liquid, physical damage or catalyst aged, the current softwares are ready to detect the faults, but providing a generic and inaccurate diagnostic.The SCR catalyst aging, due to the high level of sulfur in the diesel, to the lubricant oil burning or to the high exhaust gas temperature, will be verified in real applications when the SCR system is implemented. When it is aged, the main feature of the catalyst (the ammonia storage capacity) is reduced, which can cause ammonia emissions (slip) in the ambient. The treatment of this particular feature can also be found in sophisticated software strategies, but applying a theoric factor to the catalyst aging based on the system operating conditions to be applied as a correction factor in the closed-loop dosing control strategy.This work consists in the development of a software algorithm to be integrated to the OBD system that allows diagnosing, through a dynamic functional test, a NOx conversion efficiency reduction in the SCR catalyst as an isolated component of the complete system. With this diagnostic, applied within the strategies for catalyst aging detection, the dosing controller can correct the reagent liquid injection, preventing ammonia emissions in the ambient, and getting a faster and easier fault resolution.