The paper describes the principle of operation of ESSU and the possibilities of its diagnostic system, and additionally discusses other systems that greatly facilitate diagnostics related to troubleshooting, planning preventive maintenance, and replacement or reconstruction of worn devices to reduce the number of malfunctions of signaling and safety devices. to increase the efficiency of rail transport. The first part of the paper describes the basis of the system is ESSU SimisW, which cooperates with the Vicos interface (Human-machine interface). The ESSU has implemented a diagnostic computer, an archive computer, a computer for recording and reproducing events for monitoring traffic safety devices, and a Scada system designed to take over all control and management of part of the accompanying equipment with the ESSU, which it is not directly related to security. This is followed by a description of the installed devices, and an overview of the maintenance of individual elements of the SS device at the Zagreb Central Station and ways of monitoring their operation and diagnostics. Diagnostic instructions are given in which the malfunction on certain elements of the device is stated, as well as the possible causes of certain malfunctions and possible corrections during their elimination. In the last part, a specific example shows the diagnostic procedure related to correct and preventive maintenance on the ESSU.