This paper summarizes thermal modeling work performed on the Motorola Carbon Monoxide (CO) chemical sensor. Gas sensors need low cost reliable packages, good thermal operation, and low power consumption. The goal is to provide a validated thermal model of a gas sensor and its package and develop a sensor design capability with reduced design cycle time. Due to the complex structure of the sensor package, Computational Fluid Dynamics (CFD) tool was used to analyze the heat transfer and fluid flow within the package. Based on the validated model, parametric studies on filter location and package orientation were performed. In order to minimize the influence of humidity, the sensor is toggled between high and low temperatures by applying 5 volts for 5 seconds of heating, and 1 volt for 10 seconds of cooling. Transient thermal analysis was also performed to predict the temperature response of various components. A detailed description of the thermal model and its results are described in the paper.