The temperature of various systems must be reported to the engine control module (ECM) as well as other operating system controllers in the vehicle. Coolant temperature, cabin temperature, exhaust temperature and ambient temperature are just a few systems reported by a temperature sensor. The temperature sensor is fed 5 volts by the systems computer, within the sensor housing will be a thermistor, or dropping resistor whose resistance value will change as the temperature surrounding it changes. Changing resistance in the circuit will change the volt signal at the processor indicating the current operating value.
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