<div class="htmlview paragraph">THIS PAPER describes the use of sensitivity functions to investigate the sensitivity of engine control systems to unknown variations in parameters. The definition of output sensitivity functions and the derivation of their defining differential equations are reviewed. A structural method for generating a sensitivity model is presented and applied to a second order example. Finally, the sensitivity model for EGR valve gain of an eighteen state linear perturbation engine model is generated and the resulting sensitivity functions used to predict how engine response changes for changes in the gain of the EGR valve. These predictions are then shown to be accurate by comparison with simulations of the engine model for various values of the EGR valve gain.</div>