Radiator fan turns on when the car starts? This is why

A lot of cars do experience this problem, whose root cause many motorists simply don’t know. So, why does the radiator fan turn on when the car starts?

Well, it could be a result of a problem with your temperature gauge or sensor. When the gauge or sensor is bad, the hitch could cause your radiator fan to turn on each time you switch the ignition on. If not, there could be a problem with your cooling fan’s relay switch, switch may be stuck thus causing the fan to unnecessarily turn on when you switch on the ignition. A leaking coolant reservoir may also be another possible cause of this problem.

Why Does My Fan Turn On When I Start My Car?

A Problem With Your Temperature Gauge or Sensor

The coolant is a special fluid with characteristics that help take away the engine’s heat thereby preventing the engine from overheating.

When the liquid extracts the heat from the engine, its temperature rises as the engine’s temperature drops. To keep the rising temperatures reasonable so that the coolant remains effective, the radiator fan blows the fluid with cold air.

Now, the temperature gauge can be found on the dashboard of your car. The temperature sensor, on the other hand, is located right on the engine block or cylinder head.

The temperature gauge or sensor functions to monitor the temperature of your coolant. Unfortunately, these two systems can fail, giving the cooling fan the wrong coolant temperature information.

This can cause the fan to turn on unnecessarily when you start the car. In fact, one of the common symptoms of a bad temperature gauge or sensor is that the radiator fan turns on whenever you switch on the ignition.

A Problem With Your Cooling Fan’s Relay Switch

Your cooling fan is connected to a relay switch which quickly turns on the fan when the temperature sensor senses too much heat coming from the engine.

The relay switch does that by closing the circuit between the fan and the battery, providing electrical energy to power the fan.

Unfortunately, the relay switch sometimes gets stuck in the closed position. This constantly powers the fan, making sure that it runs provided the engine is running.

Meaning when you turn off the ignition, the engine will stop running and so will the fan. And when you turn on the ignition, the engine will start running and so will the fan.

A Leaking Coolant Reservoir

The engine start process is complex, involving the ignition, the starter motor, as well as the pistons, and the crankshaft. In order for the engine to start, the pistons and crankshaft must move in certain motions to complete a single, stroke cycle.

This process happens in seconds and generates a lot of heat reaching up to °F(700°C) or more. Fortunately, the engine’s cooling system which is made up of a special fluid known as “coolant” is there to suppress and lower these temperatures, preventing the engine from overheating.

The coolant is stored in the coolant reservoir, a small container that is normally located on the upper right-hand side of the engine. Since the reservoir is made out of plastic, it’s prone to cracks or punctures due to natural wear and tear, and other risk factors.

A cracked or punctured coolant reservoir will leak coolant, compromising the engine’s overall cooling system. As such, the engine will overheat as soon as the ignition system is turned on.

Consequently, the heat sensors will notice this problem and quickly tell the fan to turn on.

How to Fix a Cooling Fan that Goes On Right After the Car Starts?

  • Replace the gauge or sensor. A professional mechanic will charge you between $150 and $193, inclusive of labor and parts.
  • If the cooling fan relay switch is stuck, and if you are handy with your car’s electronics, you can unstick it on your own and avoid spending a dime.
  • If not, you can have a professional mechanic do it for you. The process should take about 15-20 minutes. Since the average mechanic rates per hour are $45-$170, you can expect to pay anywhere between $18.75 and $49.59.
  • Replace your coolant reservoir. Again, you can do that on your own or have a professional do that for you. Similarly, this process should take approximately 15-20 minutes. A new reservoir plus the required parts will set you back $355 and $362. Labor, on the other hand, will cost you anywhere between $59-$74.


The radiator fan is designed to start functioning only when the engine is overheated. If your fan turns on as long as you turn on the ignition system, it means there’s a problem with your cooling system.

You could be having a faulty temperature gauge or sensor, a stuck relay switch of your radiator fan, or a leaking coolant reservoir.

Fortunately, you can successfully fix these issues on your own or have your mechanic do that for you if you aren’t handy with your car’s cooling system.

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