Engine coolant temperature sensor (ECT)

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By Reinier (Contact Me)
Last Updated 2021-07-23
Automobile Repair Shop Owner

What Does the Engine coolant temperature sensor (ECT) Do?

SPECIAL NOTE: This article deals with conventional vehicles, as opposed to hybrid and/or electric vehicles. These vehicles can have multiple separate and discrete cooling systems for different components apart from the internal combustion engines’ cooling system on for instance, a hybrid vehicle. In combination, the cooling systems on hybrid vehicles can have upwards of a dozen coolant temperature sensors, multiple coolant pumps, and up to thirty or more different coolant hoses. END OF SPECIAL NOTE.

Engine Coolant Temperature sensors monitor and measure the temperature of the engine coolant continuously for two main reasons. The first is to provide the driver of a vehicle with a constant and accurate indication of the engine’s temperature, while the second is to regulate, initiate, or terminate several critical engine and emission control functions throughout the engine’s allowable temperature range.

Why is the Engine coolant temperature sensor (ECT) Needed?

In practice, the temperature of an engine is a critical factor in efficient engine operation. This is particularly true for gasoline engines since the combustion of a gasoline/air mixture is very inefficient at low temperatures. With this in mind, let us look at some of the primary functions of Engine Coolant Temperature sensors in some detail, starting with-


On modern gasoline engines, the PCM (Powertrain Control Module) can increase, or decrease the volume of fuel the injectors inject into the cylinders under some conditions. This is known as the “Injection pulse width”, and typically lasts between about 1 millisecond, to about 1.5 milliseconds per injection event. Thus, the longer the pulse width becomes, the more fuel the injector injects since the injector stays open for longer.

Since combustion is very inefficient when an engine is cold, the PCM uses input data from an Engine Coolant Temperature sensor to increase the injectors’ pulse widths to enrich the air/fuel mixture, which has the effect of improving the combustion process. The enriched air/fuel mixture, in combination with appropriate changes to the ignition timing, ensures both a stable idling speed and acceptable engine power delivery while the engine is still cold.

As the engine warms up, the engine transfers its heat to the engine coolant and since the Engine Coolant Temperature sensor is immersed in the coolant, the sensor transmits the changing temperature to the PCM. When the engine coolant reaches a pre-defined threshold, the PCM reduces the injectors’ pulse width, since the engine’s heat is now sufficient to sustain stable combustion.

By extension, stable combustion improves fuel economy and reduces harmful exhaust emissions, so one could think of Engine Coolant Temperature sensors as integral components of the larger emission control system.

Engine cooling

As the engine continues to warm up, the thermostat also opens* to allow the engine coolant to circulate freely throughout the engine’s cooling system. However, to regulate the engine’s temperature within a very narrow range, the PCM uses input data from the Engine Coolant Temperature switch to activate the radiator cooling fan(s) when the coolant temperature approaches or exceeds a maximum allowable threshold.

*Note that on engines with mapped thermostats, the PCM directly controls the opening and closing of the thermostat, as opposed to the thermostat reacting solely to the temperature of the coolant.

In a fully functional cooling system, the PCM will keep the cooling fans in operation until input data from the Engine Coolant Temperature sensor “tells” the PCM that the engine coolant temperature has fallen below the maximum allowable threshold*, at which point the PCM will deactivate the cooling fan(s).

* Note that on engines with mapped thermostats, the PCM can control the radiator cooling fans independently of the coolant temperature or inputs from the any Engine Coolant Temperature sensor to increase or decrease the engine’s temperature quickly.

However, some engine cooling systems are a little more complicated than the example described here. On some applications, there may be several Engine Coolant Temperature sensors, each of which has a specific purpose. Here are some details-

Radiator fan switch

These switches are similar in both construction and operation to engine-mounted Engine Coolant Temperature sensors. However, unlike engine-mounted sensors, dedicated radiator fan control switches/sensors are located in or near the radiator, where they directly monitor the temperature of the coolant flowing through the radiator, as opposed to monitoring the temperature of coolant flowing through the engine. When such a sensor senses that the coolant’s temperature is approaching the maximum allowable level, it will activate the radiator cooling fans without necessarily involving the PCM.

Glow plug control

Diesel engines are fitted with glow plugs, which serve as heat sources to initiate the combustion process of the air/fuel mixture during cold engine start-ups. While a dedicated control module controls the glow plugs on most modern diesel engines, this control only extends to how power is supplied to each glow plug.

In practice, since the glow plug control module does not have the means to measure the temperature of the cylinders, it uses input data from a dedicated Engine Coolant Temperature sensor to determine if the glow plugs need to be activated.

Generally, glow plugs are activated only if the engine coolant’s temperature is below a minimum threshold. Thus, if the engine coolant’s temperature is below this threshold, the glow plug control module will activate the glow plugs to start the engine, and deactivate the glow plugs when the coolant’s temperature reaches or exceeds the minimum allowable temperature threshold.

As a practical matter, the PCM on a diesel vehicle will check and verify the coolant temperature via the Engine Coolant Temperature sensor that controls the glow plugs before it will complete the starting circuits. This is to prevent activation of the glow plugs when the engine is hot, which could potentially cause severe damage to both the glow plugs and the engine when hot glow plugs initiate the premature or uncontrolled combustion of the air/fuel mixture.

How Does the Engine coolant temperature sensor (ECT) Work?

In their simplest form, Engine Coolant Temperature sensors are “thermistors”, which are devices whose electrical resistance changes in direct response to changes in temperature.

In the case of Engine Coolant Temperature sensors that have two wires, one wire carries a 5-volt current that is supplied by the PCM. This current is known as a “reference voltage”, which has to pass through a resistive element in the sensor before it is passed back to the PCM. As a rule in cold engines, the electrical resistance of the sensor is high, and only a small fraction of the reference voltage is passed back to the PCM.

However, as the engine coolant warms up, the resistance of the sensor decreases, and progressively more current is passed back to the PCM, which interprets the changing current as changing temperatures. On modern vehicles, the PCM will share this information with the module that controls the instrument cluster, which will display the changing temperature on a gauge much like how a fuel gauge indicates changing fuel levels.

However, some Engine Coolant Temperature sensors have three, or even four wires. In these designs, the basic operating principles of the sensor remain the same, but the additional terminals are used to activate radiator cooling fans, glow plugs, dedicated warning lights, or in some cases, EGR (Exhaust Gas Recirculation) systems.

Where is the Engine coolant temperature sensor (ECT) Located on the Engine?

This image shows the location (arrowed) of the Engine Coolant Temperature sensor on a Ford Taurus engine.

As mentioned elsewhere, however, some applications may have multiple coolant temperature sensors, so we strongly recommend that you research the possible locations of all coolant temperature sensors on your particular vehicle before you condemn any particular sensor.

Typically, though, Engine Coolant Temperature sensors are located on or near the thermostat housing as shown in this example, or somewhere on the cylinder head(s). Other locations could be in either the top or the bottom radiator tanks, or in a special fitting in a radiator hose.

What Does the Engine coolant temperature sensor (ECT) Look Like?

This image shows an example of a typical Engine Coolant Temperature sensor. Note that while all Engine Coolant Temperature sensor designs follow this general pattern, these sensors are application-specific, and are therefore NOT interchangeable with sensors that may appear similar, or even identical to the original sensor.

What are the Symptoms that the Engine coolant temperature sensor (ECT) is Bad?

The most common symptoms of bad or failing Engine Coolant Temperature sensors are largely similar across all applications, and could include one or more of the following-

  • Stored trouble code(s) and illuminated warning light(s)
  • Inaccurate, fluctuating, or implausible temperature readings displayed on the engine temperature gauge
  • Increased fuel consumption and elevated exhaust emission levels as the result of a faulty, defective, or malfunctioning Engine Coolant Temperature sensor not communicating with the PCM. In these cases, the PCM may not reduce injector pulse widths after a cold engine start-up. In these cases, the engine may run roughly, may not idle, or varying degrees of power loss may be present under some operating conditions
  • Engine overheating on systems where the Engine Coolant Temperature sensor controls the radiator cooling fans. Note that in these cases, the engine might overheat in a matter of seconds, which almost invariably results in fatal engine damage
  • Extended cranking times or damage to starter motors and batteries on diesel vehicles if one or more Engine Coolant Temperature sensors fail to communicate with the PCM and/or glow plug control module. In some cases, this could result in a no-start condition that will persist until the fault is found and corrected
  • Since bad or failing Engine Coolant Temperature sensors have the potential to affect or increase exhaust emissions, one or more Readiness Monitors may not run or may not run to completion, which could prevent the vehicle from passing a compulsory emissions test

How do you test the Engine coolant temperature sensor (ECT)?

Since Engine Coolant Temperature sensors work on the principle of electrical resistance, it is possible to test such a sensor by measuring its electrical resistance at pre-defined temperature points. However, while this is possible to do on a DIY basis, the different calibrations of application-specific resistance values mean that there is no single or even a broad range of resistance values that apply to all or even most Engine Coolant Temperature sensors.

Put differently, this means that unless you have access to reliable, OEM level service information with which to compare your findings, you cannot draw diagnostic conclusions on any test results- regardless of what hundreds of internet experts say on the subject. Here are two examples-

Typically, Engine Coolant Temperature sensors intended for use on most late model Ford applications will show resistance readings of 95 000 Ohms at 32 degrees F, and 2 300 Ohms at 200 degrees F. By way of contrast, consider the fact that late model GM Engine Coolant Temperature sensors will typically have resistance values of 10 000 Ohms at 32 degrees F, and 200 Ohms at 200 degrees F.

The above examples should make it clear that DIY testing of Engine Coolant Temperature sensors is useless unless you can compare your findings with OEM-level service information. Therefore, we strongly recommend that you seek professional assistance with proper diagnosis and repair of the issue if you suspect that you have a defective or malfunctioning Engine Coolant Temperature sensor.

Put simply, engine cooling system diagnostics is one area where “close enough is NOT good enough”, and making a mistake here will almost certainly cause severe, if not always fatal engine damage.

How do you replace the Engine coolant temperature sensor (ECT)?

In most cases, replacing an Engine Coolant temperature sensor is as simple as disconnecting the electrical connector, and screwing the old sensor out of the engine. Simply screw in the replacement, and reconnect the electrical connector securely.

In some cases, such as on many BMW and VAG-group vehicles, the sensor does not screw into the engine but is held in place by a spring clip that must be removed before the sensor can be removed from the engine.

However, in all cases, it is important to allow the engine to cool down, preferably overnight, before you remove an Engine Coolant Temperature sensor. Removing the sensor from the engine while it is hot could release large volumes of extremely hot coolant and/or steam, both of which could cause serious injuries.

Note also that you need to locate and identify the suspected defective sensor correctly since some applications have a) sensors that closely resemble Engine Coolant Temperature sensors, and b), may have multiple coolant temperature sensors at different points in the engine cooling system.

Also, be sure to check the engine coolant level, and to top off the level as required after an Engine Coolant Temperature sensor replacement to ensure efficient and proper operation of the engine cooling system.