Why Radiator Fan Always On?


Your concern isn’t out of place if your radiator fan continues to run after you turn off your car’s engine. The good news is, it’s a common occurrence with some types of vehicles and may not necessarily indicate any serious problem. Still, it’s important to understand why your cooling fan stays on longer than necessary. 

A radiator fan that’s always on likely has a serious issue preventing it from stopping, which is best diagnosed by a mechanic. If a radiator fan continually runs long after the engine is off, it’s commonly due to a faulty cooling fan relay or electronic control unit, or the radiator needs cleaning. 

In the rest of this article, I’ll explore the main reasons for your car’s radiator fan to keep running. I’ll also explain the seemingly odd behaviors of a radiator fan that are, actually, perfectly normal. Plus, I’ve provided a quick way to fix this rather common problem.

Is It Normal for Radiator Fans to Always Run?

It’s normal to expect some quiet immediately after turning off your car’s ignition. And it’s still okay if the fan runs for a couple of minutes. The engine might be too hot, and the radiator fan is just doing a little extra work to prevent overheating. 

However, one of the first signs of trouble with your car’s cooling system is a radiator fan that continues to run when the engine is off. If the fan keeps humming for up to ten miniatures or more, you likely have a serious problem, as car fans shouldn’t operate this way. (It even gets worse when you hear the loud noise from across the street!)

It’s not normal for radiator fans to always run, even when the engine’s temperature is high. Continuously running can shorten the lifespan of the radiator fan. For this reason, the configuration of cooling fans allows them to cycle on and off only when the need arises. 

Apart from the problem of short lifespan, if your fan constantly runs, it can drain your car’s battery, requiring you to jump-start your car the next time you need to use it. Yet that pales in comparison to shelling out for a new car battery! 

If your radiator fan is always running, it’s best to avoid such hassles by tackling the problem head-on. Or, I should say, by having a mechanic tackle the problem. You may have an electrical fault at hand, and that’s not something you want to ignore or necessarily try to fix yourself. Generally speaking, a professional mechanic should always inspect your car’s cooling system if it’s malfunctioning. 

Read: Car Full Service Cost // Whats included?

Why Is My Car Fan Still Running When the Car Is Off?

Your car radiator fan still running when the car is off is likely due to a broken or faulty fan relay. Other common reasons for the problem include a faulty chip in the electronic control unit (ECU) and a failing coolant temperature sensor (CTS). 

Beyond this, there are several common reasons for why radiator fans keep running when the engine is off. Fortunately, there are some things you can do to remedy this situation, apart from an experienced mechanic examining your car, that is.

Problem With the Cooling Fan Relay

Most modern cars come with relays that control the cooling fans. Without getting all too technical, the function of the fan relay is to switch the power supply to the engine’s cooling fan on or off. 

If the relay is bad, the fan can’t regulate the engine’s temperature. 

But that’s not all. 

A faulty relay may continuously supply power to the fan, even when the engine is already at the ideal temperature. That’s because the relay is constantly in the “on” position. 

Ultimately, this is why the radiator fan keeps running with the ignition off. 

Read: How to Tell If a Relay Fuse Is Blown

Faulty Electronic Control Unit (ECU) Chip

Think of the ECU as a command center of some sort – it controls fuel injection, fuel supply, ignition, and air management. The ECU chip receives signals from the coolant temperature sensor (CTS) and turns the fan on or off, according to the temperature signal. 

If water gets into the chip, things can go out of whack with the software inside the ECU. The software will constantly give a false command. 

If this ever happens, it can make the fan keep working long after the car’s engine is off. 

Read: Car Diagnosis Cost

Failing Coolant Temperature Sensor (CTS)

Another component that can mess with the normal operation of the car’s radiator fan is the CTS. The function of the sensor is to monitor the operating temperature of the car’s engine. 

If this system fails, it can send the wrong sensor information to the control unit (ECU). The false signal then keeps the radiator fan in continuous work mode. 

While this is a simplistic explanation of the communication process between the CTS and ECU, it should give you a fair idea of why your radiator fan keeps running. 

Read: Temperature Gauge High But Car Is Not Overheating

Other Reasons Radiator Fan Always Runs

A few other reasons may cause the radiator fan to keep working longer than necessary. These include: 

  • Faulty fan controller module. Some vehicles come with a separate module that controls only the radiator fan. The module can fail and cause issues with the fan. This problem is especially common in cars where the fan controller module’s location exposes it to dust and corrosion. 
  • A dirty radiator. The radiator may need a thorough cleaning if the fan is always on when the car air conditioning (AC) is also on. 
  • Bad or broken wiring. Wrong wiring or broken wiring can cause a short circuit. Observe the insulation on the fan wires to check for signs of frays or damages. 

How To Stop Radiator Fan From Running?

Now, here’s the quick and easy solution I promised at the start of this guide. 

If your radiator fan is always on, the likely suspect is the cooling fan relay. 

Fixing a fan relay issue is not complex, and anyone with basic car maintenance experience can do it. All you have to do is follow these simple steps:

  1. Look inside your engine’s bay fuse box to find the cooling fan relay. The diagram of the fuses and relays on the box will easily help you locate the fan relay. 
  2. Remove the relay and test it to make sure it’s still working appropriately. 
  3. Replace the relay if it’s bad. 

If changing the relay doesn’t fix the problem, the ECU may be telling the relay to remain in the “on” position. 

Sometimes, a damaged wire may cause a short circuit and mess with the radiator fan’s operation. In this situation, all you need to do is locate the problem spot and then fix the wire. 

However, if none of the above solutions work, it’s likely that you’re dealing with something more technical. Should this be the case for you, it’s usually best to have your mechanic check out the possible cause and make necessary repairs. 

Regardless of the underlying reason for a continuously running radiator fan, it’s crucial to correct the problem as quickly as possible. The sooner you find a solution, the sooner you avoid risking further damages, more repairs, and of course, spending more money down the line.

Final Thoughts

It isn’t normal for radiator fans to constantly run for extended periods, lest they die off too quickly and even drain your car battery. 

If you have some car repair skills, then you can follow the steps in this article to fix the issue yourself. However, it’s often best to get a professional mechanic to check out and address this problem, and I strongly recommend you do so, especially if you don’t consider yourself the handy, DIY type. Though, hiring a mechanic can also be helpful if you simply don’t have the time to work on the car yourself.

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