For small engines in non-automotive powertrains the emissions share is already limited. The introduction and integration of ECU-systems for engine control and dedicated sensors in small engines are required. The ion current sensing technology could be a key enabler for next generation combustion diagnoses and maintenance of small engines. It is an add-on sensing system and aims on gaining knowledge about the combustion process from the ion current signal. In this paper we present an analysis of an ion current sensing system from a metrological point of view. We investigate the impact of the ignition system and the ion current sensing system on the ion current signal and calculate a measurement error. We present a potential parameter to characterize the combustion process independently of the instrumentation. The analysis represents a first approach on how to design robust ion current based ECU control systems.