In this paper, we investigated the possibility of using of defect modes (DMs) in the second photonic bandgap (PBG) in defective one dimensional (1D) photonic crystals (PCs) for creating optical sensors. The dependencies of the relative sensitivities on the defect layer (DL) thickness for the first and second PBGs at optimized PC parameters are obtained and compared and the advantage of the first PBG over the second PBG in all ranges of the DL thickness is shown. However, as the order of the defect mode (DM) and optical contrast of the structure increase, this relative advantage becomes less prominent. The behavior of the DMs and their relative sensitivity outside the PBG when DL thickness changes are considered. The dependences of the relative sensitivity on different parameters of the PC for DM in the first and second PBGs are also compared. This work is mainly theoretical and aims at finding general patterns that can be generalized to any specific examples and parameters of 1D PC with DL for any practical realizable sensors.