One of the most puzzling phenomena observed at the surface of sdB stars is their abundance nomalies. For instance, high-resolution optical spectroscopy shows that the C abundance is ower than the solar abundance by about a factor of ten on average. A few stars, however, show a olar abundance or an abundance higher than solar. Although optical spectroscopy allows the measurement f relatively high C abundance, it fails to provide stringent upper limits when no C linesare detected. The Far Ultraviolet Spectroscopic Explorer (FUSE) comes to the rescue when no Clines are detected in the optical, because it is 1,000 times more sensitive than the VLT for the purposeof measuring very small amounts of C in the atmospheres of sdB stars. We measure the Cabundance in the FUSE spectra of 30 stars. We confirm the C abundance trend reported in previousstudies, but report the disappearance of C in five stars. The upper limits that we measure for thesestars are about four orders of magnitude lower than the solar abundance. The observation of verylow C abundance suggests that the normal equilibrium between gravitation and radiative levitation has been disrupted, perhaps by a weak stellar wind.