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To test the reliability of the SmartFire® ionization data as a parameter for detecting misfires, we fitted an inline-4 cylinder engine with in-cylinder pressure sensors and compared their data to that provided by ionization current sensing. The ignition circuitry was tested for its ability to recognize misfires, and for its potential to re-strike the ignition early enough in the cycle to provide smooth engine operation. Unique design features of the SmartFire ignition circuit enable the measurement of ion current as early as 300 microseconds after the initiation of the spark discharge, thereby allowing early detection of improper combustion. We compared the indicated mean effective pressure (IMEP) to the integrated ionization signal. When a misfire was measured by the IMEP, the integrated ion signal indicated the misfire as well. The ignition circuitry successfully identified the cycles with improper combustion.