###### Abstract
The emissions from the vehicles have been regulated for RDE testing using the Portable Emissions Measurement System (PEMS) during in-use field operations for on-highway diesel vehicle engines and model year designations [9,10]. In general, on-board diagnostic systems (OBD) monitor the vehicle's real-time running status to test the real-time NOx emissions under real-world driving conditions. The OBD system has a significant role in monitoring vehicle emissions and improving the reliability of the aftertreatment devices. In recent years, many researchers have focused on studying the OBD system. [PERSON] et al. [11] designed a model-based control strategy of OBD for the lean NOx trap aftertreatment system, and OBD diagnosed the faults such as the sensor fault, system ageing, sulfur poisoning by comparing the predicted values with the measured data. [PERSON] et al. [12] realized rapid and accurate diagnosis of SCR catalyst aging based on the OBD NOx sensor, and the control strategy corrected the urea injection amount as the catalyst ageing was diagnosed. [PERSON] et al. [13] designed the OBD strategy of the SCR catalyst based on the NOx sensor and achieved the Euro V emission standard through bench calibration. [PERSON] et al. [14] proposed the OBD control strategy based on respectively NOx and NH\({}_{3}\) sensor monitoring and verified the OBD strategy for the urea-SCR system by a diesel engine test on a test bench. Therefore, OBD can effectively monitor the real-time emissions from the vehicle during on-road driving. Moreover, a large number of real-road driving emission test results [15,16] have shown that, even if the diesel vehicle equipped with an SCR system worked, the NOx emissions were still far higher than the legal requirements. For example, [PERSON] et al. [17] conducted emission tests on thousands of in-use heavy-duty diesel trucks on a main highway in San Francisco and found that the NOx emissions of most trucks exceeded the limits of the current emission standards. The International Committee on Clean Transport assessed the actual NOx emissions of 160 in-use heavy-duty diesel vehicles in the United States, and they reportedthat when the vehicles were running at a speed of less than 40 km/h, the NOx emissions were more than five times the emission standard [18]. [PERSON] et al. [19] tested the heavy-duty diesel and natural gas vehicles equipped with, respectively, three different SCR models, and they revealed that the NOx emissions of these vehicles were five to seven times higher than the standard during transportation near the dock and in the local area. These above publications revealed the phenomenon of NOx emissions exceeding the standard limit even for those vehicles equipped with the SCR system. On the one hand, the reasons for this phenomenon are due to the ageing of the SCR system and its catalytic performance deterioration. The design of the system may not be adequate for all driving conditions or an auxiliary emission strategy may be triggered. On the other hand, one important reason for this phenomenon lies in the failure diagnosis, for example, the exhaust temperature sensor. Once the temperature sensor has a certain mechanical or electronic failure, or even cheating by artificial tampering, the effectiveness of the SCR system greatly decreases. In China, the NOx emission control for diesel-engine vehicles has been a big issue due to their huge vehicle numbers. [PERSON] et al. [20] investigated the total NOx emissions for the Beijing heavy-duty diesel vehicles (HDDV) fleet and estimated the total increasing NOx emissions for the Chinese HDDV fleet. Furthermore, the total NOx emissions increase with the age of the in-use diesel vehicles. It is necessary to understand the NOx emission characteristics of in-use diesel vehicles as they run on the real road. To check in-use vehicles exceeding the emission standard level, in-use diesel vehicles are required to stop for emission inspection at many roads check stations in China.