Operations of the on board diagnostic (OBD) system, is directed to ensuring the correct engine work in respect of the exhaust gases toxicity. Thus, when the parameters analysed by this system are within the permissible values, the system controls them in such a way as to obtain the optimal working conditions from the point if view of the exhaust gases toxicity. The user can be completely unaware of any problem occurring until exceeding of the limiting values causes switching on the MIL lamp or the transfer into the failure mode operation or even engine shut-down. The previous investigations indicated explicitly that there is such group of defects (mainly mechanical), at which the OBD system performing self-regulation does not provide the user with the information concerning a defect and, in addition, weakens typical external symptoms such as e.g. power drop. Current works of the authors performed for petrol engines indicated that there is representation between VA signal symptoms and mechanical defects, which at their initial phases are not detected by the OBD system. Thus, the authors decided to verify the sensitivity to VA signals of Diesel Engines.