reducing the overpressure the ptc probe or sensor 44 is located in a second vertical passage 28 that communicates with the horizontal passage 24 b this probe 44 is housed in a central bore of a support body 46 which is screwed into a tubular retaining screw 48 which is in turn screwed into a threaded part at the lower end of the second vertical passage 28 of the body 22 of the device 16 seals 45 47 and 49 are provided between 1 the probe 44 and its support 46 2 the support 46 and the retaining screw 48 and 3 the screw 48 and the body 22 the probe 44 has an enlarged top part that extends at its upper end upwards beyond the support 46 so as to be swept by the fuel flowing from the fuel source towards the engine at its lower end the ptc probe 44 is provided with a thinned rod connected by electrical conductors 50 to the electronic circuit 52 the electronic circuit 52 controls the intensity of the current flowing through the ptc probe 44 and detects a variation in the electrical resistance of the ptc element caused by an increase in its temperature when it is no longer swept by a flow of liquid fuel detection of the change in electrical resistance of the ptc probe 44 immediately causes a visual signal 54 or an audible signal 54 to be issued at the vehicle 39 s dashboard fig4 shows an example of the electronic circuit 52 according to a first embodiment the electronic circuit 52 is supplied via the power supply 56 for the modules of the circuit 52.