Why does the car’s cooling fan keep running?

The situation when the car radiator cooling fan keeps running constantly can be caused mostly by a failing coolant temperature sensor or its wiring, broken fan start relay, damage to the electrical wires, and electronic engine control unit (ECU) problems.

To understand how the cooling fan should work correctly, you need to know at what temperature the electronic control unit (ECU) turns de radiator cooling fan on. Another way to find out is to look at the data on the fan enable sensor located in the radiator. Usually, it is in the range of + 87 and + 95 ° C (188 and 194 F).

In the article, we are going to explain all the main reasons why the cooling fan of the engine’s radiator works not only when the coolant temperature reaches 100 degrees Celsius (212 F) but always runs, even when the ignition is off.

So why is the cooling fan constantly running?

If the cooling fan keeps running, even when the engine is off, this could be due to the following reasons:

Failing coolant temperature sensor or damage to its wiring

If the incorrect information goes from the sensor to the ECU (an overstated or understated signal, its absence, a short circuit), then errors are generated in the ECU, as a result, the electronic control unit switches the engine into emergency mode, in which the fan “works” constantly so that there is no overheating engine. 

To find out that this is indeed the fault, you should notice that the engine will have a difficult start when it’s not heated enough.

Short circuit to mass wires

Often, the fan constantly runs if its negative wire is frayed. Depending on the engine, the wires can be located in different places. 

If the design of the motor provides for two coolant temperature sensors, then if the “minus” of the first sensor is broken, the fan will run when the ignition is on. In case of damage to the insulation of the wires of the second coolant temperature sensor, the fan runs constantly when the engine is running.

Faulty fan relay

For most cars, the fan power supply consists of “plus” from the relay and “minus” from the ECU in terms of temperature from the coolant temperature sensor. “Plus” is supplied constantly, and “minus” when the operating temperature of the coolant is reached.

Problems with the electronic control unit (ECU)

The incorrect operation of the ECU can be caused by a malfunction in its software (for example, after a flashing) or if moisture gets inside it. As moisture, there can be a banal coolant that got into the ECU.

Dirty radiator

This applies to both the main radiator and the air conditioning radiator. In this case, the fan often runs constantly when the AC is on.

The freon pressure sensor in the AC

If it fails and the refrigerant leaks, the system “sees” that the radiator is overheating and tries to cool it down with the fan that is always on. 

For some drivers, when the air conditioning is turned on, the cooling fan runs constantly. This should not be so since this indicates either a clogged (dirty) radiator, or problems with the freon pressure sensor (freon leak).

Low efficiency of the cooling system

Breakdowns can be associated with a low level of coolant, its leakage, a bad or failing thermostat, pump failure, depressurization of the radiator cap, or expansion tank. With such a problem, the fan may not work continuously, but for a fairly long period or it will often turn on and off.

What to do if the cooling fan is constantly running

When the engine cooling fan runs constantly, it is worth looking for a malfunction by doing a few simple diagnostic steps. Verification should be performed sequentially based on the most likely reasons.

Check for errors in the ECU memory

For example, the p2185 error code indicates that there is no “minus” on the coolant temperature sensor, and several others (from p0115 to p0119) indicate other faults in its electrical circuit.

Check the integrity of the wires

Depending on the design of the motor, individual wires connected to the fan drive may be damaged (insulation is usually damaged), which causes a short circuit. 

Therefore, you just need to find the spot where the wire is damaged. This can be done either visually or with a multimeter. 

Alternatively, insert two needles into the contacts of the tokens and close them together. If the wires are intact, the ECU will give an error of motor overheating.

Check the coolant temperature sensor

When everything is in order with the wiring and power supply of the sensor, then it is worth checking the coolant temperature sensor. 

Along with checking the sensor itself, it is also necessary to check the contacts on its chip and the quality of chip fixation, maybe some of its clips are broken. If necessary, clean the contacts on the chip from oxides.

Relay and fuse test. Check if power comes from the relay to the fan using a multimeter (the PIN can be found in the diagram). There are cases where it “sticks”, then you need to change it. If there is no power, check the fuse.

Cleaning the radiators and cooling system

If the main radiator or the radiator of the air conditioning is covered with debris, they need to be cleaned. The engine radiator can also be clogged inside.

If that’s the case, then it is necessary to clean the entire cooling system with this special solution. Or dismantle the radiator and wash it separately.

Check the operation of the cooling system

The fan can operate continuously with the low efficiency of the cooling system and its elements. Therefore, it is advisable to check the cooling system, and if faults are detected, repair or replace its parts.

Checking the level of freon and the operation of the refrigerant pressure sensor

To carry out these procedures and eliminate the cause, it is better to visit a workshop.

Checking the EC

is an extreme measure when all other components have already been checked. In general, the electronic control unit must be removed and its body disassembled. 

Next, check the condition of the internal board and its elements, if necessary, clean it with alcohol from coolant and debris.


In summer, driving with a cooling fan that runs frequently is undesirable, but acceptable. However, if the radiator’s cooling fan runs the same in winter, it is recommended to diagnose and fix this fault as soon as possible.

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

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