(ECT) – Engine Coolant Temperature Sensor-Function, Failure And Testing

Engine Coolant Temperature Sensor-(ECT)
Engine Coolant Temperature Sensor-(ECT)

The engine coolant temperature sensor-(ECT) is a sensor that is used to monitor the temperature of the engine’s coolant.

Most (ECT) sensors operate using electrical resistance to measure the temperature of the coolant.

The readings from the (ECT) sensor are sent back to the engine control unit (ECU).

The engine control unit (ECU) uses this data to adjust the fuel injection and ignition timing.

Engines require more fuel when they are cold, and less fuel when they are fully warmed up.

The (ECT) sensor is one of the most important engine management sensors.

Its readings play a key role in calculations which affect engine performance.

The most common symptom that indicates a bad (ECT) sensor is an engine control system that fails to go into closed loop once the engine is warm.

Other Signs of a Failure:

  • Poor fuel economy
  • Black smoke from engine
  • Overheating engine
  • Check Engine Light comes on

The OBD II system should catch the fault and turn on the Check Engine Light or Malfunction Indicator Lamp (MIL).  You may see one of the following diagnostic trouble codes:

Engine Coolant Temperature Sensor-(ECT)
Engine Coolant Temperature Sensor-(ECT)

P0115….Engine Coolant Temperature Circuit
P0116….Engine Coolant Temperature Circuit Range/Performance
P0117….Engine Coolant Temperature Circuit Low Input
P0118….Engine Coolant Temperature Circuit High Input
P0119….Engine Coolant Temperature Circuit Intermittent

Input from the (ECT) sensor may be used by the PCM for any or all of the following control functions:

  • Start up fuel enrichment on fuel injected engines. When the PCM receives a cold signal from the (ECT) sensor, it increases injector pulse width (on time) to create a richer fuel mixture. This improves idle quality and prevents hesitation while the cold engine is warming up. As the engine approaches normal operating temperature, the PCM leans out the fuel mixture to reduce emissions and fuel consumption. A faulty (ECT) sensor that always reads cold may cause the fuel control system to run rich, pollute and waste fuel. A (ECT) sensor that always reads hot may cause cold driveability problems such as stalling, hesitation and rough idle.
  • Spark advance and retard. Spark advance is often limited for emission purposes until the engine reaches normal operating temperature. This also affects engine performance and fuel economy.
  • Exhaust gas recirculation (EGR) during warm-up. The PCM will not allow the EGR valve to open until the engine has warmed up to improve driveability. If EGR is allowed while the engine is still cold, it may cause a rough idle, stalling and/or hesitation.
  • An open thermostat or the wrong thermostat may be preventing the coolant from reaching its normal operating temperature.

  • Evaporative emissions control canister purge. Fuel vapors stored in the charcoal canister are not purged until the engine is warm to prevent drivability problems.
  • Open/closed loop feedback control of the air/fuel mixture. The PCM may ignore the oxygen sensor rich/lean feedback signal until the coolant reaches a certain temperature. While the engine is cold, the PCM will remain in “open loop” and keep the fuel mixture rich to improve idle quality and cold driveability. If the PCM fails to go into “closed loop” once the engine is warm, the fuel mixture will be too rich causing the engine to pollute and waste gas. This condition may also lead to spark plug fouling.
  • Idle speed during warm-up. The PCM will usually increase idle speed when a cold engine is first started to prevent stalling and improve idle quality.
  • Transmission torque converter clutch lockup during warm-up. The PCM may not lockup up the torque converter until the engine has warmed up to improve cold driveability.
  • Operation of the electric cooling fan. The PCM will cycle the cooling fan on and off to regulate engine cooling using input from the (ECT) sensor. This job is extremely important to prevent engine overheating.

Testing Made Easy

(ECT) Connector
(ECT) Connector

Keep in mind that many (ECT) sensor problems are more often due to wiring faults and loose or corroded connectors than failure of the sensor itself.

A visual inspection of the (ECT) sensor will sometimes reveal a problem. This could be severe corrosion around the terminal, a crack in the sensor, or coolant leaks around the sensor.

In most cases, the only way to know if the (ECT) sensor is good or bad is to measure its resistance and voltage readings.

The First Thing To Check Is the ECT Sensor Wire Harness-(Using A MultiMeter)

This allows you to check if the harness that receives the signals have a problem in relaying the signal to the computer. This will rule out any faulty wiring.

  1. Pull out the harness connected to the sensor to reveal the connection point between the harness and the sensor.
  2. Turn the ignition key on without starting the engine.
  3. Connect the red clip to terminal two while grounding the black clip.
  4. If the MultiMeter reads at most 5 volts then the harness has no problem.


  1. Start with a cold engine. With the ignition off, disconnect the wiring connector from the (ECT) sensor.
  2. Attach an ohmmeter across the sensor’s terminals.
  3. Measure the sensor’s resistance and record the reading.
  4. Reconnect the sensor wiring connector.
  5. Start and run the engine for two minutes and then shut the engine off.
  6. Disconnect the sensor wiring harness again, and take an ohmmeter reading across the sensor’s terminals.
  7. Compare the two readings. There should be a difference of at least 200 ohms. If not, the sensor is defective.


The reference voltage to the sensor from the PCM should be about 5-volts. The return voltage signal should be around 3 to 4 volts when the engine is cold. This should gradually drop to 2 volts or less as the engine reaches normal operating temperature. No change in the return signal would indicate a faulty (ECT) sensor.


Even general wear and tear can cause the sensor to erode over time.

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