## 5 Summary  
In this paper, a feasibility study for transferring On-Board Diagnosis (OBD)-related calibration and validation tasks from the vehicle to a Hardware-in-the-Loop (HiL)-platform is presented. As a case example, an active diagnostic for monitoring the condition of the three-way catalytic converter (TWC) and the oxygen sensors (UEGO and HEGO sensors) in the exhaust aftertreatment system is used. In order to evaluate this respective transferability, three typical phases of an OBD reference project have been considered: OK system testing (Phase #1), not OK (nOK) system testing (Phase #2) and final emission verification (Phase #3). For every phase, the simulation quality of the most important signals was assessed:  
* Phase #1--All simulated physical signals required to release the respective diagnostic event (release conditions) show good agreement with real vehicle measurement data, resulting in the diagnostic event occurring at approximately the same time during a WLTC drive cycle on both the HiL and the vehicle. It was also shown that during the diagnostic run on the HiL platform, all relevant characteristics of the most important sensor signals (UEGO and HEGO sensor signals) were modelled with sufficient accuracy and led to valid diagnostic results over the entire operating range. * Phase #2--The respective fault patterns for both aged TWC and latent HEGO sensors are well represented on the HiL platform, which likewise leads to valid diagnostic results over the entire operating range. * Phase #3--The relevant gaseous emission results at the tailpipe are well represented in the HiL-simulation in comparison to real vehicle measurements, both for a new system and for aged components. The simulation accuracy obtained (simulation vs. vehicle measurement) is as follows: 1. NO\({}_{\text{x}}\) emission +31% (6%-point with regards to the EU6d legal limit) considering an OK-system and \(-\)11% (\(-\)4%-point with regards to the EU6d legal limit) considering a nOK system. 2. CO emissions \(-\)6% (\(-\)2%-point with regards to the EU6d legal limit) considering an OK-system and \(-\)4% (\(-\)1%-point with regards to the EU6d legal limit) considering a nOK system. 3. HC emissions +25% (3%-point with regards to the EU6d legal limit) considering an OK system and +45% (+5%-point with regards to the EU6d legal limit) considering a nOK system. Ultimately, it is demonstrated that all relevant signals are simulated with sufficient accuracy to ensure a robust transfer from the vehicle to a HiL platform. Based on the results obtained, it is concluded that transferring OBD work from the vehicle to the HiL test bench could lead to a significant saving in development time and that the number of prototype vehicles and test resources required can be significantly reduced. Conceptualization, F.D. and C.L. ; methodology, F.D. and C.L. ; software, P.B. ; validation, F.D. and P.B. ; formal analysis, F.D. ; investigation, F.D. and P.B. ; resources, M.N. ; data curation, P.B. ; writing--original draft preparation, F.D. ; writing--review and editing, M.T.D., S.K., C.L. ; visualization, F.D. and M.T.D. ; supervision, S.P. and M.N. ; project administration, F.D. ; funding acquisition, M.G. and S.P. All authors have read and agreed to the published version of the manuscript. **Funding:** The presented research was carried out at the Center for Mobile Propulsion (CMP) of RWTH Aachen University, funded by the German Science Council 'Wissenschaftsrat' (WR) and the German Research Foundation 'Deutsche Forschungsgemeinschaft' (DFG). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The full data setis not publicly available due to confidentiality reasons. On request, defined parts can be shared.