Engine Coolant Temperature Sensor (ECT)


General description 
      The engine coolant temperature sensor is temperature-variable resistor, which usually has a negative temperature coefficient. It is a two-wire thermistor immersed in coolant and measures its temperature. The onboard computer uses the signal of ECT as the main correction factor when calculating the ignition advance and the injection duration.


The ECT sensor is shown on fig. 1.

Fig. 1

Working principle of the ECT sensor 
      In order to convert the ECT resistance variation to voltage variation, which is further processed by the ECU, the ECT sensor is connected in a circuit typically supplied with a reference voltage of +5V. In cold engine and an ambient temperature of 20 ºC the sensor resistance is between 2000Ω and 3000Ω. After the engine start, coolant temperature begins to rise. ECT gradually heats and its resistance reduces proportionately. At 90 ºC its resistance is in the range of 200Ω to 300Ω. 
Thereby, a coolant temperature dependent variable voltage signal is send to the onboard computer.

Types of ECT sensors

  • With negative temperature coefficient. These are the most popular sensors used in cars. Their resistance decreases with the increase of the temperature.
  • With positive temperature coefficient. Used in some older systems, such as Renix. Here the voltage and the resistance are increasing with the raise of the temperature.

Procedure for verification the functionality of the ECT sensor
 Sensor with negative temperature coefficient 
Test with voltmeter

  • Open the protective rubber cap on the connector of the cooling system temperature sensor.
  • Connect the negative wire of the voltmeter to the chassis ground.
  • Determine which are the signal and the ground terminals.
  • Connect the positive wire of the voltmeter is to the ECT signal terminal.
  • Start the engine form cold state.
  • Depending on the temperature, voltage readings should be in the range of 2V to 3V. The relationship between the voltage and the temperature is shown in Table-1.
  • Check whether the ECT voltage signal corresponds to the temperature. You will need a thermometer for the purpose.
  • Start the engine and warm it to the operating temperature. During engine warming the voltage should decrease in accordance with the values given in Table-1.
  • Common problem is that the output resistance (and voltage) changes incorrectly beyond its normal range. The normal value of the ECT sensor voltage is 2V at cold engine and 0.5V at warm engine. A defect sensor can indicate a voltage of 1.5V at cold engine and 1.25V at warm engine, thus causing difficulties of starting a cold engine and a presence of rich fuel mixture when the engine is warm. This will not generate any fault codes (if the onboard controller is not programmed to detect voltage changes) since the sensor continues to operate within its design parameters. If such defect is found, the ECT sensor must be replaced.
  •  If the ECT voltage signal is equal to 0V (lack of power supply or there is a short circuit to the ground) or if it is 5.0V - we have an open circuit.

Temperature, ºС

Resistance, Ω

Voltage, V


4800 - 6600

4.00 – 4.50



3.75 – 4.00


2200 - 2800

3.00 – 3.50





1000 – 1200

2.50 – 3.00






2.00 – 2.50


270 - 380

1.00 – 1.30




Broken circuit

5.0 ± 0.1

short circuit to the ground


Table 1
NOTE: This is a typical example, but this does not mean that the values shown above are real and must be obtained in the process of verification of a specific system.

Possible sensor failures:
-- The voltage of the ECT signal terminal is equal to 0V.

  • Check sensor terminals for short circuit to ground.
  • Check the integrity of the signal wires between the sensor and the onboard controller.
  • If all wires are correct, but there is no output voltage from the onboard controller, you have to verify all power supply and ground connections of the onboard controller. If supply voltages and grounds are good, the onboard controller itself falls under suspicion.

-- The voltage of ECT signal terminal is equal to 5.0V
The voltage has such a value when an open circuit is present and may be obtained in one of the following conditions:

  • the signal terminal of the ECT sensor does not provide connection to the sensor;
  • sensor circuit is open;
  • sensor’s ground circuit is open.

-- The voltage signal or the reference voltage is equal to the car battery voltage.
Check short circuit in the wire, connected to the positive terminal of the car battery or the power supply wire.

 Sensor with negative temperature coefficient 
Check with ohmmeter with ECT sensor detached from the car

  • Place the sensor in a suitable water container and measure the water temperature.
  • Measure the sensor’s resistance and compare it with the values given in Table-1, showing the relationship between the resistance and the temperature.
  • Heat the water and take measurements periodically as well as the resistance of the sensor. Compare the results with those in table 1.

 Sensor with negative temperature coefficient 
Voltage measurement with oscilloscope

  • Connect the active end of the oscilloscope probe to the signal terminal of the sensor and the ground probe - to chassis ground.
  • Set the trigger sweep of the oscilloscope in a continuous measurement mode (registration of slowly changing signals).
  • Place the sensor in a suitable heated water container.
    After a few minutes of measurement, during the water heating time, oscilloscope screen will show the sensor’s voltage change curve (Fig. 2). Pay attention to the measuring time - it is about 10 minutes.
  • It is desirable to continuously measure the temperature of the heated water with a thermometer and compare it with the values listed in table 1.

Fig. 2

Sensor with a positive temperature coefficient  
      ECT sensor with a positive temperature coefficient of resistance is a thermistor whose resistance increases with the rising of the temperature. It is used in a small number of systems (mainly in Renault cars).
      The overall method of verification is similar to the method for checking a sensor with negative temperature coefficient, described above. Received measurement data can be compared with the data given in Table 2 showing the relationship between the resistance and the temperature of the sensor.

Temperature, ºС

Resistance, Ω

Voltage, V


254 - 266



283 - 297

0.6 – 0.8


383 - 397

1.0 – 1.2

open circuit

5.0 ± 0.1

short circuit to ground


Table 2