What to do if the cooling fan turns on when the engine is cold

What to do when the cooling fan is on when the engine is cold

At times, car owners are faced with a situation where the cooling system fan turns on and starts working when the engine is cold. Although under normal conditions of functioning of this element, it is obliged to activate only when the engine temperature reaches certain values.

Cooling fan comes on when the engine is cold

There are many reasons for such a situation in modern cars. Some of them can lead to serious negative consequences, if measures to eliminate the causes and provoking factors are not taken in time.

To understand the situation, you need to learn about the causes of the fan activation, to understand its work and find the best methods for solving the problem.


Before trying to determine why cooling system fans can turn on on a cold engine in cars, it is recommended to understand the essence of the work of this node.

The fan turns on due to the work of a special sensor that activates the rotation of the blades. This controller is located at the bottom of the cooling radiator.

There are some modern cars that the engine control unit is responsible for starting, although this is rather a rarity and the exception and rules.

When the cooling fluid (coolant) reaches the required temperature, which is usually between 100-105 degrees Celsius, the sensor closes the electrical circuit, and it allows the fan to turn on. It starts blowing on the radiator. This achieves a better cooling effect because it combines the air and the liquid system.

However, there are situations when the engine is still absolutely cold and does not need any additional cooling, but the fan is still running. This is not a normal condition. It is necessary to search for the reason of such behavior of the cooling system, and to eliminate the arisen malfunctions.

If you continue to ignore the constantly running fan, the initial simple discomfort will be replaced by more serious problems. But here it is important to note that the constant operation of the blades is not as dangerous as a complete failure of the device. Although this fact is absolutely not a reason not to do anything.

Everything starts with diagnostics. The most simple and convenient in this regard are injector engines. To detect a malfunction, it is enough to carry out computer diagnostics. This is done both in the conditions of the car service center, and in the garage with your own hands.

Diagnosing the fan problems

Connection to the electronic control unit gives the opportunity to get error codes, which are not difficult to decipher and understand the true causes of such behavior of the cooling system fan.

There are situations when the ECU detects a malfunction in the cooling system, and forces the fan to turn continuously, even when the engine is cold. This is a kind of protective function to prevent possible overheating of internal combustion engine. Therefore, when errors are detected and after troubleshooting, a reset is always made. To erase the errors, you should look in the manual of your car. In some cars it is enough to remove the terminal from the battery, and the errors will be erased. In other cars, however, the errors are erased with the help of connected diagnostic equipment.

If computer diagnostics will not suit you, or it cannot be used for some reasons, you will have to check all potential sources of trouble by method of elimination.

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Common malfunctions

Since the fan is spinning, you have to make it stop and turn on only when it’s really needed.

There are several main reasons why cooling system fans can turn on when the engine is cold. It has to do with:

  • the on-off sensor;
  • the cooling fluid;
  • the mass;
  • thermostat;
  • temperature sensor;
  • contacts;
  • air conditioner.

Each of these items may be the reason why the blower is triggered and starts spinning when the engine is cold.

Trigger Sensor

One of the most common reasons why the fan runs on a cold engine is a short circuit of the controller, that is, the sensor.

Sensor of fan activation

Quite often it happens that the fan starts at the same time as the ignition is turned on. If this happens at the same time, the contacts on the controller are probably short-circuited. As a result, the blower works all the time.

Although the breakdown is the most widespread, it is also the most harmless and safe. The worst consequence of such a phenomenon is considered a complete discharge of the battery. This will happen if you leave the ignition on for a while.

To check whether the short circuit is to blame or not, you will need to use a multimeter to check the resistance by placing the stylus on the leads. When the sensor is faulty, the multimeter shows infinite resistance, that is, it displays the corresponding value on the display.

If you see that the resistance is lower, the pins are probably shorted. You will need to replace the controller. Such a malfunction is shown by a separate code during computer diagnostics.

Cooling Fluid

Sometimes it is not necessary to look for all sorts of reasons why the cooling fan suddenly turns on the cold engine. Experienced motorists advise starting with a trivial check of the coolant fluid.

The thing is that quite often the sensor reacts by activating the fan to the lack of coolant in the system. That amount of antifreeze, which is available, heats up too quickly, which leads to the fan activation when the motor is still cold.

That is why not so rare cause of such malfunction is banal inattention of a car owner. Some drivers simply forget that it is necessary to control fluid level and periodically pour fresh antifreeze into the expansion tank. It will evaporate gradually, that is why it is necessary to refill it. When the radiator is half-empty, the sensor will really start reacting accordingly. To cool the engine, it will activate air cooling, realized with the help of a fan.

Due to the lack of coolant, it heats up faster than with a fully filled system. And the coolant temperature readings do not match the engine temperature. As a result, it turns out that the engine has just started, it has not yet warmed up, and the fan is already rotating its blades at full speed due to the high temperature of the quickly warmed up fluid.

Antifreeze level in the expansion tank

To solve the problem is very simple. Fill the expansion tank with the missing amount of antifreeze. You may also need to pump the system if there is air lock inside.

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Another potential culprit in the occurrence of such a situation with the cooling system fan is a short circuit to the ground.

Causes of such a phenomenon are quite a lot. But often it is explained by the fact that the ground wire is simply badly screwed to the car body. Because of this, the fan turns out to be directly connected to the battery, which provokes its constant rotation.

The situation seems banal, but the consequences of such defect are rather serious. The car may simply catch fire. This is possible with a strong short circuit and overheating of the wiring.

Ground wire

To fix the problem, you need to check all the connections that relate to the sensor at the bottom of the radiator. Make sure all wires are intact and have good insulation on them. Also connect the wiring firmly. Use an extra layer of insulation if necessary, or replace the wires completely if they are badly worn or damaged.

Thermostat problems

On modern vehicles, there are thermostats augmented with sensors. This combination of devices makes it possible to increase the efficiency of controlling the engine cooling system. But this solution has some weaknesses.

Periodically, the thermostat sensor stops working, which causes the constant rotation of the air cooling system fan. The reason for this phenomenon is very simple. The ECU does not receive any signals from the thermostat, as a special sensor is responsible for them. As a result, electronics sets it to work in an emergency mode. For reinsurance purposes, the ECU starts the fan to prevent the engine from overheating. If such a malfunction occurs, the fan will rotate until the sensor is returned to its functionality.

Checking the sensor should not cause any special problems for the car owner. It is actually an ordinary temperature controller. If it is in good working order, it shows infinite resistance when the engine is cold. In other cases, the resistance parameters are 100-500 ohms. If a multimeter check shows deviations from these values, the controller needs to be replaced. It is a rather cheap consumable, so you will not have to spend a lot of money on repairs.

Additional sensor

Here we are talking about the air temperature sensor. It is used on some cars as an additional controller, which regulates the temperature readings of the oncoming air flows. In fact, this device monitors and reports the temperature of the ambient air that comes from the air ducts outside the car.

If the air is too hot, which is relevant during hot summer days, it can no longer be used to cool the radiator and the engine because it itself has an excessively high temperature. When the sensor detects temperature readings higher than programmed in it, a signal is sent to activate the fan. This is intended to increase the efficiency and quality of engine cooling.

Most often, a similar scheme using a sensor to measure the temperature of the outside air entering for cooling is used on foreign cars with sandwich radiators. These are special three-layer structures.

If this sensor is present, then surely in the summer on the hottest days the fan will constantly rotate and help to cool the power unit. This is considered a normal phenomenon. On such cars, it is not recommended to disable this sensor, as you with your own hands can provoke overheating of the motor with all the ensuing consequences.

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If the fan is controlled in the car through an electronic unit, you should not exclude the occurrence of problems with oxidation of contacts. Because of this their connection is disturbed and signals are not transmitted to the control unit.

Contacts in the car

Because of this, ECU perceives the absence of signals as a failure, and puts the cooling system in the emergency mode of operation. It is manifested in the form of fan activation and some changes in operation of other systems.

In order not to provoke oxidation of contacts, experienced motorists and repairmen advise to clean them twice a year. It is better to do it at the beginning of spring and autumn.

If you suddenly found that the fan began to rotate immediately after starting the ignition, although the engine is still completely cold, first try to clean the contacts. Additionally, it is recommended to use a special lubricant for the contacts. This will prolong the service life and protect against oxidation processes.


Surprisingly, sometimes the cabin cooling system is to blame for the fan when the motor is cold.

On some vehicles, the air conditioner is largely combined with the engine cooling system, and has a number of components in common. Due to a clogged radiator in the air conditioner itself, fan operation is activated. To get rid of such a malfunction, the radiator needs to be flushed.

Since we are talking about the use of two radiators (engine and air conditioner), it will be correct to flush both structures. As a result, the paired system will be able to work properly and efficiently.

Here it is important to understand that there is nothing terrible or incredibly scary about the constant operation of the cooling system fan. This is far from the most serious breakdown that can occur. Therefore, almost any tasks to restore the normal performance of the cooling system is realistic to solve with your own hands.

This does not require specialized equipment or any professional tools. It is better to start with the diagnostics in order to determine the exact causes of malfunction. If you can’t do the diagnostics, start with the simplest potential causes, and move on to the more complex ones. First, make sure there is enough antifreeze in the cooling system and there are no air locks inside. Then you can already check the contacts, mass, thermostat and change the sensor itself.

Cooling fan is running all the time

The situation when the cooling fan constantly runs can be caused by several reasons: failure of the coolant temperature sensor or its wiring, breakage of the fan start relay, damage of the drive motor wires, “glitches” of the electronic engine control unit (ECU) and some others.

The cooling fan is always on

To understand how the cooling fan should work properly, you need to know what temperature is programmed in the ECU to turn it on. Or look at the data on the fan switch sensor, located in the radiator. Usually it is in the range of +87 … +95 °C.

In this article we will consider in detail all the main reasons why the engine cooling fan does not run only when the coolant temperature reaches 100 degrees, but always when the ignition is off.

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Reasons for switching on the fan Conditions for switching on
Failure of ignition sensor or damage of its wiring Engine started in emergency mode
Short circuit of wires to “mass”. Engine running, the fan may turn on if there is contact or there is no contact
Short circuit to ground with two ignition probes Check if the engine is running (first sensor) or ignition is on (second sensor)
Defective relay on fan switch Engine started in emergency mode
“ECU glitches Different modes, depends on specific ECU
Heat release of radiator is disturbed (contamination) With running engine, in long trip
Defective sensor of freon pressure With air conditioner on
Low efficiency of cooling system With the engine started

Why does the cooling fan always run?

If the engine fan is running all the time, there may be 7 reasons.

Coolant temperature sensor

  • Failure of coolant temperature sensor or damage of its wiring. If there is incorrect information from sensor to ECU (over- or underestimated signal, its absence, short circuit), ECU generates errors, as a result ECU transfers engine to emergency mode, in which fan “flails” continuously, so as not to overheat engine. You will be able to understand that this is what is wrong with the engine when it is not yet warmed up.
  • The wires are shorted to the “ground”. It is not uncommon for the fan to run constantly if its minus wire is frayed. Depending on the design of the engine it can be in different places. If the design of the engine provides for two DTCs, then the fan will “flail” when the ignition is on, if the “minus” of the first sensor is broken. If the insulation of the wires of the second sensor is damaged, the fan runs continuously when the engine is running.
  • Defective relay of the fan switch. In most cars, the fan power consists of a “plus” from the relay and a “minus” from the ECU by temperature from the TOR. “Plus” is supplied all the time, and “minus” when the antifreeze reaches operating temperature.
  • “Glitches” of the electronic control unit . In turn, the incorrect work of ECU can be caused by a failure in its software (for example, after re-flashing) or when moisture gets inside its housing. As moisture can be banal antifreeze that got into the ECU (relevant for Chevy Cruze cars, when the antifreeze gets into the ECU through a torn throttle heating tube, it is near the ECU).
  • Dirty radiator . It concerns both the main radiator, and the radiator of air conditioner. Often the fan runs constantly when the air conditioner is on.
  • Freon pressure sensor in the air conditioner . If it fails and the refrigerant leaks, the system “sees” that the radiator is overheating and tries to cool it with the fan constantly on. Some motorists have the cooling fan running all the time when the air conditioner is on. This should not actually be the case, as it indicates either a clogged (dirty) radiator or a problem with the freon pressure sensor (freon leak).
  • Low efficiency of the cooling system . Breakdowns can be associated with low coolant level, coolant leakage, defective thermostat, broken pump, depressurized radiator cap or expansion tank. With this problem, the fan may not work all the time, but for a long time, or turn on frequently.
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What to do if the cooling fan constantly works

When the engine cooling fan is constantly running, it is worth looking for a malfunction by making a few simple diagnostic actions. The check should be performed sequentially, based on the most likely causes.

  • Check presence of errors in ECU memory. For example, error code p2185 indicates that there is no minus at fuel gauge, and some others (from p0115 to p0119) indicate other faults in its electrical circuit.
  • Check the continuity of wires . Depending on the design of the motor, individual wires associated with the fan drive may be damaged (usually frayed insulation), which causes a short circuit. Therefore, you just need to find where the wire is damaged. This can be done either visually or with a multimeter. Alternatively, you can insert two needles into the contacts of the chip and short-circuit them. If the wires are intact, the ECU will display the engine overheating error.
  • Check the sensor. When everything is OK with the wiring and power supply to the sensor, it is worth checking the coolant temperature sensor. Along with checking the sensor itself you should also check the contacts on its connector and the quality of the connector fixation (whether the lug/latch is broken). If necessary, clean the contacts on the connector from oxide.
  • Check the relay and fuse. Check if the power comes from the relay to the fan with a multimeter (pin number can be found in the diagram). There are cases when it “jams”, then it must be replaced. If there is no power, check the fuse.
  • Clean the radiators and cooling system . If the main radiator or radiator of air conditioner is covered with debris – they should be cleaned. Clogging of the engine radiator can also form inside, then it is necessary to clean the entire cooling system with special means. Or dismantle the radiator and wash it separately.
  • Check the operation of the cooling system . The fan may run continuously with low efficiency of the cooling system and its individual elements. Therefore, it is desirable to check the cooling system, and if you find faults – to repair or replace its parts.
  • Check the freon level and operation of the refrigerant pressure sensor. To carry out these procedures and to eliminate the cause, it is better to visit the service.
  • Checking ECU is a last resort when all other assemblies have already been checked. In a general case, it is necessary to dismantle the ECU and disassemble its case. Then check condition of internal board and its elements, if necessary clean it with alcohol from antifreeze and debris.

In summer it is undesirable to drive with constantly running fan, but it is acceptable. However, if the fan is constantly spinning in winter, it is recommended to diagnose and repair the failure as soon as possible.


Most often, the radiator cooling fan constantly turns because of a short circuit in the starting relay or its wiring. The other problems are less frequent. Accordingly, diagnosis should begin with checking the relay, wiring and the presence of errors in the ECU memory.

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