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

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

The engine coolant temperature (ECT) sensors function is to, monitor the temperature of the engine’s coolant.

So, most engine coolant temperature (ECT) sensors operate using electrical resistance to measure, the temperature of the coolant.

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

The engine coolant temperature (ECT) sensor is one of the most important; engine management sensors. Because, its readings play a key role in calculations, which affect engine performance. As a result, the (ECU) uses this data to adjust the, fuel injection and ignition timing.

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

Engine Coolant Temperature (ECT)
Engine Coolant Temperature (ECT)

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 Failure Include:

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

Possible Engine Codes

The (OBD II) system should catch the fault and turn on the, check engine light or (MIL) lamp.  You may see one of the following diagnostic trouble codes:

  • 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 engine coolant temperature (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. But, as the engine approaches normal operating temperature; the (PCM) leans out the fuel mixture to reduce emissions and fuel consumption.

So, a faulty (ECT) sensor that always reads cold may; cause the fuel control system to run rich pollute and waste fuel. Also, 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) valve, during warm-up

Exhaust Gas Recirculation (EGR) Valve
Exhaust Gas Recirculation (EGR) Valve

The (PCM) will not allow the (EGR) valve to open; until the engine has warmed up to improve driveability. But, if the (EGR) is allowed, with a cold engine; 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 (O2) 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. Finally, 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. Consequently, 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

Dual Engine Cooling Fan
Dual Engine Cooling Fan

The (PCM) will cycle the cooling fan on and off to regulate engine cooling; using input from the (ECT) sensor. That’s why, its job is extremely important in preventing engine overheating.

Engine Coolant Temperature (ECT) Sensor Testing Made Easy

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. That’s why, a visual inspection of the (ECT) sensor, will sometimes reveal a problem.

(ECT) Sensor Connector
(ECT) Sensor Connector

This could be severe corrosion around the terminal; a crack in the sensor or coolant leaks around the sensor. So, 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 Wiring Harness – (Using A Multimeter)

This allows you to check if the harness that receives the signals is having 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.
Digital Multimeter
Digital Multimeter

Finally, 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, then shut the engine off.
  6. Disconnect the sensor wiring harness and take an ohmmeter reading, across the sensor’s terminals.

Finally, compare the two readings. There should be a difference of at least 200 ohms. If not, the sensor is defective.

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

Voltage – Testing

  1. The reference voltage to the sensor from the (PCM) should be about 5-volts.
  2. The return voltage signal should be around 3 to 4 volts, when the engine is cold.
  3. This should gradually drop to 2 volts or less as the engine reaches, normal operating temperature.

Finally, no change in the return signal, would indicate a faulty (ECT) sensor.


Even general wear and tear can cause the (ECT) sensor to erode over time. So, the engine coolant temperature (ECT) sensor is designed to be; fully submerged in coolant. But, will not function correctly if the coolant level is low. Also, engine coolant prevents the water from boiling. And, it also lubricates the engine block and the radiator, preventing rust formation. Finally, always use the right mix of water and antifreeze in your engine. When in doubt, consult the owner’s manual.

Thank You !