Radiator Fan Runs When Engine Is Cold (Solved)

Noticed your radiator fan kicking on even when the engine’s still cold?

That can be confusing and a little concerning. Normally, the fan should only run once the engine heats up, so when it starts spinning right from the start, it might feel like something’s off.

The good news is that there’s usually a pretty simple reason behind it. Sometimes it’s totally normal. Other times it’s a small part acting up.

In this post, we’ll break down why your radiator fan runs when the engine is cold, how to check it out, and what to do about it.

Why Does The Radiator Fan Run When The Engine Is Cold?

You’re not imagining things. Sometimes the fan really does run when the engine is cold. And there’s always a reason.

Here's what might be happening:

Faulty Temperature Sensor Or Switch

One of the main reasons behind this issue is a faulty temperature sensor or switch. 

This part tells the car’s computer (ECU) the temperature of the engine. But if it starts failing or gets stuck reading a high temperature, it can cause the fan to turn on early - even when the engine is stone cold. 

Sometimes, the sensor fails slowly and sends weird readings that come and go, which makes the issue harder to pin down.

If your fan starts running right after startup and stays on longer than it should, the sensor could be the reason.

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Stuck Or Faulty Relay/Control Module

Another reason why your radiator fan runs when the engine is cold could be a stuck or faulty relay or control module.

The radiator fan relies on a relay or control module to know when to run. If that switch gets stuck in the “on” position (or the control module glitches out) the fan may run constantly, no matter what the engine temperature is.

This can even happen when the engine is off and the key is just in the "on" position. 

Relays go bad because of corrosion, heat damage, or just plain wear and tear over time. 

A sticky or shorted relay is a common and fairly easy thing to replace, but it can absolutely cause confusing fan behavior until it’s handled.

AC Or Condenser Triggering Fan Operation

This is something a lot of people miss: the radiator fan can also be triggered by your air conditioning system. 

Even on a chilly morning, if you start the car and the AC is running, or even just set to “auto”, the fan may turn on to help cool the AC condenser. This happens before the engine ever warms up. 

In fact, this is totally normal on many vehicles. If your car has automatic climate control, it might activate the fan even when it feels like it shouldn't. 

So before chasing down electrical issues, make sure your AC isn’t what’s waking up the fan.

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What To Do If Radiator Fan Runs When Engine Is Cold

Now that we know what might be causing the issue, let’s talk about how to troubleshoot it. It’s not always going to be an easy fix, but it’s something you can definitely do on your own.

Here’s how you can start figuring out what’s going on:

1 Check AC Condenser Usage

The first thing you should do is check if the AC is on. 

If it is, that’s probably why your fan is running. The radiator fan often kicks on when the AC is being used, so if you’ve got the AC running, that could be the simple answer.

To test this, just turn off the AC and see if the fan turns off with it. 

If it does, then the fan is likely just doing its job of cooling the condenser. Nothing to worry about there! However, if the fan keeps running even after the AC is off, you’ll need to dig a little deeper.

2 Inspect The Temperature Sensor

Next up, plug in a scan tool and take a look at the live coolant temperature reading. 

If your engine has been sitting overnight and the tool shows something like 200°F, that’s a red flag. The sensor is probably lying. You can also unplug the sensor and see if the fan shuts off - that can help confirm it's the issue. 

Replacing a bad sensor usually isn’t too tough or expensive, and it can save you a lot of confusion down the road. 

Sometimes, all it takes is one faulty signal to throw the whole system out of sync.

3 Test The Relay/Module

If the sensor reading looks normal, it might be time to dig into the relay. 

Check your fuse box and find the fan relay (it's usually labeled). If there’s another relay with the same specs nearby (like a horn or fuel pump relay), try swapping them and see what happens. 

If the fan starts behaving normally, bingo. You’ve found the culprit. 

For vehicles with a separate fan control module, testing is a bit trickier, but signs of wear, corrosion, or overheating are all worth checking. 

A failing relay or module can cause the fan to run when it really shouldn’t.

4 Scan For ECU Trouble Codes

Even if your dash isn’t lit up with warning lights, your car might still be holding onto some useful trouble codes. 

Use your scan tool to check the ECU for any stored codes related to the cooling system, temperature readings, or fan control. 

This can often give you a direct clue - like a P0118 code for a high voltage signal from the coolant temp sensor. Catching these early saves a lot of time. 

A simple code read can quickly point you toward the right fix instead of chasing random parts.

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When To Go To A Mechanic

So you've done the quick checks. AC is off, sensor looks fine, relay swap didn’t fix it, and the fan still runs way too early - or all the time. 

That’s when it’s time to call in a pro. 

If things aren’t adding up or if digging into relays and modules isn’t your thing, a mechanic can step in with tools that go way beyond a basic code scanner.

Modern cars rely on a lot of computer inputs to control something as simple as a fan. And if your issue is hiding in a deeper electrical problem, a flaky ECU connection, or something like a fan control module buried under the battery tray, it’s probably not worth the hassle trying to sort it alone. 

Mechanics have thermal scanners, advanced diagnostics, and experience that makes this stuff quicker to figure out.

Bottom Line

If your radiator fan is running when the engine is still cold, there’s a reason - and it doesn’t always mean something’s broken. It might just be your AC doing its thing. 

But if it’s not that, it’s probably something simple like a temperature sensor or relay.

Take a few minutes to check the basics. Turn off the AC. Use a scan tool. Look at the relays. 

Most of these fixes won’t take long and don’t cost much. And if you're not into DIY stuff, any decent mechanic should be able to track it down in no time.