In order, in a diagnostic unit for leak testing of a tank system of an internal combustion engine, to allow minimization of the energy consumption during a diagnostics test and simultaneously prevent the emission of fuel gases, a sleep/wake-up mechanism is provided. The diagnostic unit enters a sleep mode for a selected time interval after the internal combustion engine is turned off, the diagnostic unit enters a wake-up mode at the end of the first time interval and at least acquires the measurement data necessary for the diagnostics test in the wake-up mode. The diagnostic unit then enters the sleep mode again during the diagnostics period. The diagnostic unit is switched off at the end of the diagnostics period.