Abstract: A control system for determining combustion quality in a combustion chamber of an internal combustion engine. For each combustion chamber, first and second sampling windows are generated and ionic currents sampled utilizing the spark plug as an electrode. In response to the samples, indications of combustion quality such as misfire, late combustion, and slow combustion are provided. When the engine is operating in a lean burn mode, rich correction are made to the engine air/fuel ratio in an amount dependent upon the combustion quality indications.