The invention relates to a method for generating an ion current which occurs as a direct current between a central electrode and one or more side electrodes of a spark plug of a spark ignition engine which is supplied repeatedly with an ignition voltage from an ignition voltage source, wherein the spark ignition engine is assigned an engine control unit, which, in an engine cycle, determines for each spark plug a target ignition point and/or an activation point for the ignition voltage source before the target ignition point, and a second voltage source is provided, which delivers a second voltage for the generation of the ion current, wherein the second voltage source lying in the engine cycle for a first time interval Δt1, which is shorter than the duration of the engine cycle, is connected to the electrodes of the spark plug, and wherein the ion current flowing as a result of the application of the second voltage to the electrodes of the spark plug takes a path which avoids the ignition voltage source.