Why Does the Car Fan Keep Running? Thermistor Switch Failure and How to Check It

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Does your radiator fan keep running even after you turn off the ignition? Or do you often hear the fan kicking in too frequently while driving? These symptoms may point to a faulty thermistor switch. In this post, we'll explore the reasons why the fan keeps spinning, the role of the thermistor switch, common failure signs, and how to troubleshoot the issue yourself.

 

 

1. What is a Thermistor Switch?

A thermistor switch is a temperature-sensitive ON/OFF switch that activates the radiator fan when coolant temperature exceeds a certain level. In most vehicles, it’s mounted on the radiator, thermostat housing, or engine cylinder head.

A digital illustration explaining why a car fan keeps running due to a faulty thermistor switch, featuring a gray sedan with a red fan icon and warning triangle pointing to the switch location, all set against a clean, informative layout.

2. Common Reasons the Fan Keeps Running

  • Thermistor switch failure (stuck ON): Internal contacts stay closed, causing the fan to run even when not needed.
  • Wiring short or relay malfunction: A stuck relay or shorted wire can also cause the fan to stay on continuously.
  • ECU control error: In some models, the ECU controls the fan, and faulty temperature sensor data can cause abnormal fan operation.

 

 

3. Symptoms of Thermistor Switch Failure

  • The fan continues running for several minutes after the engine is turned off.
  • The fan activates suddenly even when the engine is cold.
  • Battery drains faster due to frequent fan operation.
  • The fan runs excessively when the A/C is on.

4. DIY Diagnostic Steps

  1. Locate the thermistor switch: Usually found near the lower radiator or thermostat housing.
  2. Check resistance with a multimeter: When removed and tested at room temperature, the switch should be open (no continuity).
  3. Heat test: Apply heat and check if the switch closes at the rated temperature.
  4. Test the relay: Swap or remove the fan relay to determine if the issue lies in the relay circuit.

 

 

5. Tips for Replacing the Thermistor Switch

  • Verify the temperature rating: Example: ON at 95°C, OFF at 88°C.
  • Prevent coolant leakage: Drain some coolant or use a stopper before removing the sensor.
  • Use a torque wrench: Avoid overtightening to prevent cracking the housing.
  • Bleed air after replacement: Run the engine at idle to remove air from the cooling system.

 

 

Conclusion

A constantly running radiator fan is not just a minor annoyance—it could be a sign of electrical or thermal control issues in your car. By understanding how the thermistor switch works and how to diagnose or replace it, you can prevent overheating, save battery life, and restore proper fan behavior. Identifying the cause yourself also helps streamline professional repairs if needed.