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This paper discusses the development of a system enabling real-time measurements of combustion pressure in an internal combustion engine, using a comparatively low-cost polycrystalline piezoelectric element. Tests conducted using a mass production vehicle showed that by using the measured cylinder pressure, the status of combustion could be measured across the entire range of driving conditions, enabling accurate detection of misfires and knocking. To date, despite their comparatively low cost, the use of piezoelectric elements in mass production vehicles has represented a challenge, because the temperature characteristics of the elements in the use environment, age degradation, and other factors may cause variations in output. This research therefore studied the variation in sensor output in terms of each causal factor.