Carbon dioxide automotive air conditioning (AC) systems have been under development for over a decade. Although the AC system construction is important, a reliable refrigerant leakage detection system is also vital. A detailed thermodynamic simulation model and fault detection and diagnosis (FDD) system, with proposed validation plans, has been developed at Rochester Institute of Technology. A discussion of simulation models that have been developed for various compressors and heat exchangers is presented; they are compared to actual AC systems to develop a realistic experimental setup. Assumptions from previous work are examined and improved.Portions of this paper were previously published in the ECOS'05 and ECOS'07 conference proceedings.