The coolant temperature sensor (CTS) is commonly found on modern injection engines.
It measures the coolant (water) temperature in the engine itself, it is not to be confused with the thermostat as found on car radiators. The Engine Control Unit (ECU) uses the CTS value to calculate the ignition timing and injector pulse duration. The CTS is a thermistor that measures the temperature and sends a signal back to the ECU. The CTS is immersed in the engine coolant and contains a variable resistor that usually operates on the NTC (Negative Temperature Coefficient) principle. When the engine is cold the resistance increases. As the engine warms up, so too does the coolant and the resistance of the CTS decreases. This in turn returns a variable voltage signal to the ECU based upon the coolant temperature. A faulty temperature sensor can result in any of the following, - poorer fuel economy - trouble starting once the engine is up to operating temperature or black smoke from the tailpipe as well as triggering a fault code that will light up the engine dash warning light.