The activity in the field of computer-aided optimum design in engineering of electrochemical gas sensors has been increasing steadily over the last decade. A vast range of models exist today, varying in complexity and in the number of assumptions employed. However, the emphasis in a majority of the models has been either on the transport processes or on the electrochemical processes. This manuscript presents a complete mathematical model of electrochemical gas sensors, represented as a system of the partial differential equations of parabolic and hyperbolic types and the algorithm of transfer from the complete model to models of specific sensors. A complete mathematical model shows that the physic-electro-chemical processes occurring in the electrochemical gas sensors can be described more accurately. Presented mathematical model together with the proposed algorithm provides a decision-making tool for better optimal design of the solid electrolyte gas sensors. An example of transfer from a complete model to real model of the yttria-stabilized zirconia (YSZ)-based potentiometric gas sensors has been shown for the YSZ-based oxygen sensor with Pt sensing electrode (SE) and metal-metal oxide (Me-MeO) reference electrode (RE). Verification of the adequacy of the mathematical model to real gas sensor has been evaluated by the Fisher criterion.