The paper discusses the identification of malfunctions in the EGR system based on a list of OBD II error codes. The authors monitored EGR systems of vehicles fitted with DURATORQ TDCi engines. The Analysis of the processes of heat transfer and mass exchange in the engine intake system drew attention to a number of simplifications in the formal description introducing a wide range of tolerance for the instantaneous recirculation rate. The aim of this study was to determine the influence of diesel oil, Ekodiesel Ultra with biocomponents and B10 fuel used for the fueling of diesel engines on the formation of deposits in the EGR system. The analysis was performed in a laboratory utilizing the methods of energy dispersive X-ray fluorescence and infrared spectroscopy.