an oil level sensor includes a sensor body adapted to be mounted to the block or casing of an oil - containing reservoir . a probe including a pair of electrodes forming a capacitor extends from the sensor body so that the capacitor will be at least partially immersed in the oil to detect the level thereof . at least one of the electrodes is insulated so that the apparent capacitance of the capacitor is relatively insensitive to any change in the conductivity of the oil at high temperatures . circuitry is provided in response to the capacitance of the capacitor for signalling the level of the oil , which capacitance is a function of the oil level . the sensor body is mounted to the block by external threads on the probe . the electrodes of the probe extend proximally past the threads to provide an increased capacitance for the sensor . the electrodes of the probe are concentric and the inner electrode is supported at both ends to reduce fatigue . circuitry is also provided for automatically testing the proper operation of the sensor .