## 4 Discussion  
The results presented in Figures 5-10 demonstrate the feasibility of enabling fault path verification on a HiL platform for TWC and HEGO diagnosis, including fault entry in the fault memory (MILing) and the respective fault healing. As the MILing/healing sequence executed on the HiL test bench meets the legislative requirements, the HiL-based  
Figure 14: Total simulated and measured NO\({}_{x}\), CO and HC emission results for an OK and a noK (aged HEGO sensor) system in relation to the EU6d legal limit. Figure 13: Comparison of WLTC HiL and chassis dynamometer measurement for a noK system (aged HEGO sensor)—Tailpipe emissions NO\({}_{x}\) (cumulated). methodology is able to support during Phases #1 and #2 of the OBD development process (Figure 2). To execute typical MILing/healing verification on the HiL test bench, detailed modelling of the EATS (e.g., detailed simulation of component temperature and emission behavior) is not mandatory. Interpreting this in relation to Figure A1, only a \"Level 5\" model is needed to provide HiL support with the engineering tasks defined in Phases #1 and #2 (Figure 2) of the OBD development process. The active TWC and the HEGO diagnosis are among the most complex diagnoses in terms of modelling requirements (a closed-loop HiL configuration with perfectly simulated HEGO voltage is mandatory). Therefore, it can be assumed that diagnoses with lower levels of complexity in terms of model requirements can be examined via the same method and using the same modelling environment as well. Examples of this are sensor plausibility and/or electrical diagnostics, as these often only require a HiL configuration with open-loop control. An example of a HiL-based plausibility check of temperature sensors is presented by the authors in [63]. Figure 7 shows that the model environment, for both stabilized and aged components (borderline TWC and latent HEGO sensor), represents plausible system behavior over the entire operating area. Also, a clear separation between the stabilized and the aged components is apparent. Therefore, the system set-up would also be suitable for pre-calibrating of the respective failure thresholds, and a carry-over of selected OBD calibration tasks can be ensured. The simulation results presented in Figures 11-14 show good results in terms of the simulated emission behavior compared to vehicle measurements from the chassis dyno. As emission behavior can be simulated in good agreement with chassis dyno measurements, the HiL based methodology is able to support during Phase #3 of the OBD development process (Figure 2). For HiL support during Phase #3, detailed modelling of the engine-out emissions and the emission conversion of the TWC depending on the oxygen loading and monolith temperature is mandatory, which requires a \"Level 6\" model according to Figure A1. Transferring OBD-related calibration and verification tasks from the vehicle to a HiL platform offers several advantages in terms of both flexibility and reduction of overall project time, as well as efficient use of resources:  
* The HiL platform enables a high degree of test automatization via dedicated automatization units (e.g., a failure insertion unit to simulate electrical faults). Furthermore, automated calibration can be realized using an application programming interface (API) to connect the automatization unit of the HiL platform with the calibration software (e.g., INCA V7.2). This advantage enables 24/7 testing with only limited human supervision. Furthermore, it paves the way for advanced software testing by executing test cases and sequences in an intelligent way. * The target powertrain components that are to be diagnosed can be easily replaced by corresponding failure components. Both real hardware (e.g., real throttle valve) and simulated hardware (e.g., simulated aged TWC) can be connected to the HiL platform and can be replaced quickly and automatically. With this advantage, elaborate workshop activities can be reduced to a minimum.