Mathematical model of biosensor with competitive substrates conversion is analysed in this work. Model is described by partial differential reaction-diffusion equations with non-linear reaction term. Because of the non-linearity the analytical solutions exist only for extreme parameter values and thus the model in general case is solved by finite difference methods. The validity of the computational model is checked by comparing numerically obtained results to the known analytical solutions at the mentioned extreme parameter values. The purpose of this work is to determine the values of model parameters at which the impact of one of the substrates on the biosensor response can be minimized.