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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 3.3.2.1 (requirements for the OBD system Stage 2), it is needed to monitor the catalyst efficiency as an isolated component of the system.