Double-Layer Capacitors (DLC) are becoming popular energy storage elements in several systems such as automotive propulsion systems, renewable energy storage and auxiliary power units. Optimal energy management, estimation and control algorithms for such DLCs heavily depend on sensor measurements, namely voltage, current and temperature sensors. Any fault in such sensors may result in degraded operation of DLCs thereby increasing the risk of significant damage. In this paper, an equivalent circuit model in conjunction with a set of observers is employed to design a sensor fault diagnosis scheme for DLCs. The scheme consists of an electrical and a temperature observer, the output errors of which are used as residual signals for fault diagnosis. Further, to suppress the effects of modeling uncertainties, adaptive threshold generators are also designed. Simulation studies are presented to validate the effectiveness of the proposed approach.