Radiator Fan Runs When Engine Is Cold – Causes & Fixes

Under perfect conditions, the car engine produces much heat during combustion. So, car manufacturers design a cooling system to prevent the engine from overheating. The primary purpose of the cooling system is to keep the engine at the optimal operating temperature.

However, some cooling system components are engineered to come on only when the engine is warmed up. One of these components is the radiator fan. Therefore, something is wrong if the radiator fan runs when the engine is cold.

In this article, I’ll explain the possible reasons for this problem and the best solutions. By the time you get to the end, you will be able to answer the question, “Why is my radiator fan constantly running” and be able to proffer solutions.

radiator fan runs when engine is off

What Causes the radiator fan runs when the engine is cold?

A radiator fan running when the engine is cold means there are one or more wiring issues somewhere in the vehicle. However, other issues like a stuck open thermostat, malfunctioning ECM/ECU, lousy fan relay, and faulty coolant temperature sensor can be the leading cause.

Let’s look at these issues one after the other and how they can cause to radiator fan to run constantly.

Stuck open thermostat

The thermostat is responsible for controlling coolant flow into the engine. When the engine is cold, the thermostat prevents the coolant from entering it, allowing the engine to warm.

When the engine warms up to a certain temperature, the thermostat opens and allows coolant to circulate. The coolant circulation prevents the engine from overheating. If the thermostat is stuck open, it may cause the fan relay to trigger the radiator fan even when the engine is cold.

Lousy fan relay

The cooling fan relay is a switch that supplies electric flow to the radiator fan. By doing so, it tells the fan when to blow and when not to. The relay activates the fan when the engine gets a certain temperature and also shuts down the fan when the temperature drops.

The relay can malfunction or go bad over time. If this happens, it can cause the fan not to activate at all or cause it to blow constantly, whether the engine is warm or not.

Faulty engine temperature sensor

The engine temperature sensor, also called the coolant temperature sensor is responsible for monitoring the coolant temperature and forwarding the signal to the powertrain control module (PCM). The PCM will constantly adjust the fan based on the information it receives from the coolant temp sensor.

So, a lousy engine temperature sensor will send the wrong or no signal that will cause the fan to blow constantly.

Defrost function is active.

One of the reasons the cooling fan is coming on when you start your car is that the defrost function on your vehicle is activated. The defrost function activates automatically under certain temperatures to prevent the windows from fogging. So, if the cooling fan is coming on when you start the vehicle under certain temperature conditions, check the defrost function.

Faulty fan motor or electrical issues

While it is rare for a faulty fan motor to cause the radiator fan to run constantly, it happens anyway. Remember I said earlier that the common reason for the fan to blow always is one or more wire issues.

If there is an internal wiring issue in the cooling fan motor, it may cause the fan to run continuously even when the engine is off. So, if your radiator fan runs when engine is off, you have one or more wiring issues. Contact a certified electrician to diagnose your vehicle if the cooling fan comes on with the ignition.

Lousy engine control unit

The engine control unit (ECU) is the brain of the car. It controls various functions of the vehicle, including the cooling system. It derives data from several car sensors and makes necessary adjustments with the information it receives.

The ECU can malfunction due to hardware failure or software glitches. If that happens, the ECU will show erratic behavior. If the radiator fan turns on and off constantly, examine the ECU. It could be the culprit.

Radiator Fan Runs When Engine Is Cold

How to fix the Problems?

As you have seen, there are several reasons for a cooling fan to start running once you start the engine. Hence, there are several ways to fix this problem.

Turn off the defrost function.

As explained earlier, the fan could be running even in cold weather conditions because the defrost function is on. All you need to resolve the problem is to turn off the climate control system.

Replace bad relay

If your fan relay is bad, the only feasible solution is a replacement. To replace it;

  • Use the diagram on the cover or the guidelines on the service manual to locate the fan relay.
  • Disconnect the negative battery terminal
  • Gently pull out the lousy relay
  • Instal the new relay.

Inspect and replace the thermostat.

You want to first test the thermostat to confirm if it is the root cause of the problem. The thermostat is usually close to the water pump or at the end of the upper radiator hose. Drain the engine coolant before removing the radiator hose. After that, test the thermostat and ensure it is working correctly.

Replace the lousy temperature sensor.

If the temperature sensor is faulty, it could be the culprit. Diagnose the vehicle to pull out the logged error codes. A P0116 and P0118 indicate the engine temp sensor is bad. To fix this problem, you have to locate and replace it with a new one. The temperature sensor is mounted close to the thermostat housing on the head cylinder.

Test and replace a faulty fan motor

If you suspect the fan motor is the root cause, inspect it. Start by disconnecting the negative battery terminal. After that, unplug the fan wiring harness and remove the fan from the engine bay. Get a battery and jumper cables and test the radiator fan motor using a multimeter. Change the fan motor if it is faulty, and reinstall the fan assembly.

Inspect and fix wiring issues.

Inspect the connectors and wiring harness that goes to the radiator fan. Look out for corrosion, wear, or any damage. If any part of the wire of the connectors is damaged, replace them accordingly.

Contact a certified electrician.

Some electrical issues are not the matter of armatures to deal. For instance, problems with the ECM need to be diagnosed by a certified electrician. Nonetheless, if you have tried everything I outlined above and the issues persist, the only option is to contact an electrician.

Should the radiator fan run all time

Unless you have a cooling fan connected directly to the engine via a serpentine belt and a pulley, the fan should not run all the time. A radiator fan running all the time indicates that something is wrong with one or more electrical wires or the cooling system components. But when should the cooling fan come on.

Under perfect conditions, the radiator fan should only come on when the engine temperature rises from 195 to 220 degrees Fahrenheit. The fan should also turn off once the engine temperature goes below this range.

Final Words

In summary, if a radiator fan runs when the engine is cold, you have one or more wiring issues that need attention. It can also happen due to problems like faulty fan motors, stuck open thermostats, defective temperature sensors, and lousy fan relays.

Luckily, fixing this issue only requires turning off the climate controls or replacing a lousy fan motor, fan relay, thermostat, or coolant temp sensor.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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