ECTSensor

Engine Coolant Temperature Sensor (ECT)

Group: Temperature and oil pressure

The ECT sensor measures the engine coolant temperature. It is one of the most important inputs for the computer: cold starting, mixture, idle, the cooling fan and the switch to closed loop all depend on it.

Contents

What does the ECT sensor do?

It is an NTC thermistor: its resistance drops as the temperature rises. The computer feeds it 5 V through an internal resistor and reads the resulting voltage. With a cold engine it richens the mixture and raises the idle; with a warm engine it leans the mixture, enables oxygen sensor feedback and switches the fan on when needed. If the sensor says the engine is cold when it is hot, the engine will burn a lot of fuel and blow black smoke.

Where is it located?

In the cylinder head or the thermostat housing, in contact with the coolant. Some vehicles have a second sensor for the dashboard gauge.

Signal type and reference values

  • 2 wires: signal (with a 5 V reference) and ground.
  • Approximate resistance: about 2,500 ohms at 20 °C (68 °F) and 200 to 300 ohms at 90 °C (194 °F).
  • Approximate voltage: 3 to 3.5 V cold and 0.5 to 1 V with the engine warm.
  • Normal operating temperature: 85 to 105 °C (185 to 220 °F).

Values are a guide only: always check the manufacturer's data for your engine.

Symptoms of a bad ECT sensor

  • Hard starting when cold or when hot.
  • High fuel consumption, black smoke and a smell of gasoline.
  • The cooling fan does not come on (overheating) or runs all the time.
  • Erratic temperature gauge.
  • Codes P0115 to P0119 or P0128.

How to test the ECT sensor step by step

  1. With a cold engine (after sitting overnight), compare the ECT and IAT temperatures on the scan tool: they should match within 3 to 5 °C.
  2. Start the engine and watch the temperature climb: it should rise steadily to 85 to 105 °C and then stabilize.
  3. If it stays below 80 °C (176 °F) while driving, suspect a thermostat stuck open (P0128) before the sensor.
  4. Unplug the sensor and measure its resistance at a known temperature; compare it with the manufacturer's chart.
  5. Check for the 5 V reference at the connector and look for corrosion on the pins.

Trouble codes related to the ECT sensor

Frequently asked questions about the ECT sensor

Are the ECT sensor and the gauge sender the same thing?
On many modern vehicles, yes: the computer sends the reading to the instrument cluster. Others have two separate sensors, one for the computer and one for the gauge.
Can a bad ECT sensor make the engine overheat?
Yes, if it stops the cooling fan from switching on. Watch the temperature gauge and do not keep driving if it climbs above normal.

Updated September 25, 2026. Values are a guide only: always confirm them in the service manual for your engine.

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