The uncertainties introduced during some stages of the micro-fabrication process can introduce a variation in the thermal response and calculated sensitivity of micromachined gas sensors. In this paper a parametric study is conducted to investigate the effect of the variation in the thermal characteristics and the electrical characteristics (induced at different levels of the micro-fabrication of CMOS thin films) on the sensitivity of gas micro sensors. Three dimensional finite element modeling was used to conduct the parametric study. The multilevel substructuring modeling technique was used to reduce the computational cost of the analysis. The used substructuring technique is proven to reduce the computational cost of the parametric analysis by a factor that ranges from 57.8 to 78.7%. This expensive computational task has shown that the variation of some material properties can alter the sensitivity of gas micro sensors by a factor that can reach up to 40%. This large variation in sensor performance highlights the essential need for a close monitoring of different parameters involved in various micromachining stages of the gas micro sensors.