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A sensor inside the combustion chamber, reading the full pressure trace of every firing stroke — the one instrument that watches combustion directly, where misfire detection, knock sensing and oxygen sensors all infer it from side effects. Engines that carry them run closed-loop combustion control: injection timing and quantity trimmed per cylinder, per cycle, against what actually happened rather than what should have. On diesels the sensor is typically built into the glow plug — a pressure-sensing glow plug — so replacing one is a glow plug job at several times a plain plug's price, which is worth knowing before a quote reads as padded. Their codes also hand the workshop a free diagnostic: with fuel cut on overrun, every cylinder's trace should be identical, so one sensor disagreeing with its siblings has convicted itself.