The current very rapid technological development of means of transport with the accompanying high economic demands gives rise to their modernization, which is technically and financially particularly advantageous. One of the dominant processes in this reality is modernization of diesel locomotives that seeks, in particular, to improve their operating characteristics, energy, environmental and also maintenance parameters. This paper deals with diagnostics and troubleshooting on a modernized diesel-electric locomotive series 757 ZSSK, a.s. We present details on this traction vehicle diagnostics and specific diagnostic files that originated in real operating conditions, with the subsequent solving of diagnostic problems and tasks.