In this work, first results are presented of the analysis of the behaviour, in actual, non-new operational conditions, of a lean burning, natural gas (NG) fueled, 1 MWe Caterpillar (CAT)-3516 engine, installed at the Engineering Faculty of Perugia University for the cogeneration of electrical and thermal energy. Data collected daily by the plant manager operators along 24 months of engine operation, which refer to power, electrical efficiency, NG flowrate, availability, and charge temperature, were analysed in connection with the recorded events of breakdown/defects and of maintenance/repair. An acquisition system for the continuous monitoring of several engine and plant quantities is described in detail.