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Due to the independent operation of the NOx (Nitrogen Oxides) aftertreatment systems from the engine, the aim of the OBD system (OnBoard Diagnosis) is to guarantee the NOx and the Particulate Matter emissions stay within the emissions standards during all the vehicle useful life. In the case of heavy duty diesel vehicles, that use the SCR system (Selective Catalytic Reduction) as the NOx aftertreatment technology, the use of the NOx sensor in the exhaust pipe will be indispensable to control and to monitor the engine emissions. In parallel with these advances in the engine embedded technology for reduction of emissions, it is also advancing the use of biofuels such as biodiesel, which appears as a renewable and clean alternative because it is derived from organic mass and results lower concentrations of some pollutant gases in the exhaust gas in comparison to the fossil fuel.

Due to the independent operation between the aftertreatment systems and the engine, the aim of the On Board Diagnostic System (OBD) is to ensure the engine emissions stay within the emissions standards during the whole vehicle useful life. In the case of the heavy duty diesel vehicles that use the Selective Catalytic Reduction System (SCR) or the Exhaust Gas Recirculation System (EGR) as the NOx aftertreatment technologies to meet the stringent emissions levels, the use of sensors in the exhaust pipe is required to control and to monitor the engine emissions. These are new and great challenges to the national diesel engine developers who are working with these systems to get the homologation certification. Accurate mathematic models within the automotive control strategies are becoming ever more important and are strongly used to monitor the NOx emissions directly (in case of SCR systems using the NOx sensor) or indirectly (in case of EGR systems using the Lambda sensor).