Engine coolant temperature sensor

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The coolant temperature sensor is used to measure the temperature of the engine coolant of an internal combustion engine. The readings from this sensor are then fed back to the Engine control unit (ECU), which uses this data to adjust the fuel injection and ignition timing. On some vehicles the sensor may also be used to switch on the electric cooling fan. The data may also be used to provide readings for a coolant temperature gauge on the dashboard.

Types of sensor

As the sensor's temperature changes, its resistance changes accordingly. Depending on the type of sensor, the resistance will either increase or decrease.

  • In negative temperature coefficient (NTC) sensors, the internal resistance will decrease as the temperature rises (and vice versa).
  • In a positive temperature coefficient (PTC) sensor, the opposite is true. Its resistance will increase with rising temperature.

Most automotive coolant temperature sensors are NTC sensors.


The ECU sends out a regulated reference voltage (typically 5 volts) to the coolant temperature sensor. The voltage drop across the sensor will change according to the temperature because its resistance changes. The ECU is then able to calculate the temperature of the engine, and then (with inputs from other engine sensors) uses lookup tables to carry out adjustments to the engine actuators, i.e. change the fuel injection or ignition timing.

This is necessary because in order to run smoothly, a cold engine requires different timing and fuel mixture than an engine at operating temperature.

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