This apparatus includes means for measuring capacitance or resistance and means for simulating a selected capacitance value. The apparatus is connectible into a fuel gaging system. When so connected, a selector switch can be operated to make the normal connections between the fuel gaging capacitors and the indicators, so that the indicators produce their normal responses. Alternatively, the selector switch may be operated so that the apparatus measures the capacitance or resistance of various elements in the gaging system. As another selection, a simulated value of capacitance may be inserted in the gaging system to make the indicators in that system respond to the simulated value in addition to the capacitance of the fuel gaging capacitors of the gaging system.