Optimization of the fuel combustion process in a self-ignition engine with the multi-stage HPCR injection system sets the main trends in research on the thermodynamic stability of fuels, and the mechanisms of PM formation. The stages were indicated of the multifaceted diagnostic inference on the causes of failures of a turbocharger with variable geometry (VTG) which occur as a result of PM sedimentation in the nozzle area. An evaluation of the engine performance was conducted using a dedicated tester and an additional recording of the injector coil current characteristics with parallel readings of the fuel pressure variation in the reservoir. The indicated procedure established the underlying cause of the VTG compressor failure despite the absence of a recorded error code.