This paper first presents a basic mean value engine plant model implemented in a hardware-in-the-loop (HIL) system. The plant model includes some basic engine parameters such as engine speed, manifold absolute pressure, etc., which are critical to both control algorithm integrity and default actions that result from improper signal performance (e.g., ECU shuts down due to corrupted signal(s)). The model is then improved to develop the HIL bench-based testing capabilities in the areas where a vehicle has traditionally been required. The on-board diagnostic monitor tests covered by SID $06 of SAE J1979 are selected as a case study. Specifically, for OBD exhaust gas sensor monitor testing purposes, the oxygen sensor model is developed to simulate normal or abnormal binary switching signals which might have asymmetric “lean to rich” and “rich to lean” transitions, or largely off maximum/minimum sensor voltages, etc. For a variable valve timing (VVT) system, a cam phaser model is developed which enables the HIL system to simulate normal cam phasing as well as phasing system errors such as slow response, cam signal stuck, etc. Verification of catalyst monitoring is made possible through integration of the ASAP3 protocol into the model to accomplish the capability for the testing script to accurately synchronize the front and post oxygen sensor signals with the ECU's air/fuel ratio command. The basic ideas behind the models will be explained in the paper. With the HIL-based test bench running the newly-developed engine plant model, we have developed the capability to automatically conduct OBD exhaust gas, VVT, and catalyst monitor tests, which are normally done in vehicle. Some test results will be demonstrated in the paper as well.