A low-cost fiber-optic pressure sensor is reported designed for use in production automotive engines for combustion monitoring and control. The sensor operates on the principle of changing light intensity due to the reflection oflight from a metal diaphragm moving under the effect of combustion pressure. Two designs are described: one based on the use of a single, multi-mode fiber and the other using two multi-mode fibers. The optoelectronic components used in the sensor include one GaA1As LED operated at 850 am and one or two SI PIN photodiodes. To combat LED intensity and photodiode sensitivity changes over an underhood temperature range of -40°C to 150°C a proprietary auto-referencing technique has been developed that does not require temperature regulation of the optoelectromc package. Telecommunication-grade optical fibers, with core diameters ranging from 50 to 100 microns, are used coated with high temperature coatings. A sensor, as small as 2.8 mm in diameter, can be either directly inserted into an engine head or integrated with a spark plug or a fuel injector. The sensor optoelectromc and electronic components are packaged inside a miniature "smart" connector that connects to an engine controller power supply as well as outputs pressure and sensor health signals. When optimized for high linearity and reduced thermal effects, the spark plug-mounted sensor demonstrates accuracy comparable to that of a laboratory-grade piezoelectric transducer. Under engine operating conditions ranging from idle to full load and maximum RPM, the combined sensor's hysteresis, non-linearity, and thermal shock result in pressure reading accuracy of 1.5% full scale range. With the targeted life-time of 500 Million pressure cycles and price of less than $10, the sensor meets the performance, durability, and cost requirements for production car applications.