Coolant sensor

What does the coolant sensor do: Its usually fitted at the cylinder head, some cars can have more then one sensor. The coolant sensor is used to monitor the temperature of the engine coolant inside the engine. The sensor showes the temperature in a resistance change as it does in proportion between colder to warmer coolant. Input from the coolant sensor tells the computer when the engine is warm so the ECU can change the opening time of the injectors and they function. It uses the coolant temperature to know when to run rich or lean the engine needs to be according the engine temperature.
Cooland sensor diagnosis: A visual inspection of the coolant sensor will sometimes have a problem such as corrosion around the terminal, a crack in the sensor or coolant leaks around the sensor. But in the most cases, the only way to know if the coolant sensor is good or bad is to measure its resistance and voltahe reading. The internal resistance of the coolant sensor can be measured and checked with an ohmmeter and compared to the specification. If the sensor is open, shorted or reads out of range, it must be replaced by a new one as there is a fault in it or at the wiring.
Coolant sensor voltage checks: You can also use a voltmeter to check the sensors output. Specs vary on the sensors and brands, but normally a cold coolant sensor will show something around 3 volts. As the engine warms up, and reaches operating temperature, the voltage drop should go down to 1.2-0.5 volts. Best to start the engine and wait for 5 minutes, it will reach its operating temperature.