Thermistor Switch Replacement Guide: DIY Car Repair You Can Do Yourself

If your car’s radiator fan is behaving strangely—running nonstop or not turning on at all—one component you should suspect is the thermistor switch. This small part controls when the cooling fan turns on and off based on engine temperature. In this post, we’ll walk you through how to locate, replace, and test a thermistor switch yourself, along with tips, tools, and safety precautions to make it a successful DIY repair.

 

 

1. When Should You Replace the Thermistor Switch?

  • Radiator fan keeps running after engine shutoff
  • Fan turns on even when the engine is cold
  • Fan does not turn on, causing engine overheating
  • Engine warning light or cooling fan control DTCs (Diagnostic Trouble Codes)
    An instructional digital illustration showing the thermistor switch replacement process, including a gray sedan with an arrow indicating the switch location, a gloved hand using a wrench to remove the old switch, and a comparison between the old and new thermistor switches inside a circular inset.

2. Where to Find the Thermistor Switch

The switch location varies by vehicle, but it is commonly found in:

  • Lower or side section of the radiator
  • Near the thermostat housing
  • On or around the cylinder head

 

 

3. Tools and Materials You'll Need

  • Replacement thermistor switch (make sure it’s compatible with your vehicle)
  • Wrench or socket set
  • Torque wrench (recommended)
  • Coolant (some or all may need to be replaced)
  • Drain pan or catch tray
  • Shop towels and gloves

4. Step-by-Step Replacement Procedure

  1. Let the engine cool down completely to avoid burns.
  2. Drain some coolant via the lower radiator hose or drain valve.
  3. Unplug the thermistor switch connector and remove the old switch with a wrench.
  4. Install the new switch and tighten to the proper torque (typically 15–20 Nm).
  5. Reconnect the plug and refill coolant.
  6. Start the engine and bleed air from the cooling system (let it idle until bubbles stop).
  7. Check for leaks and ensure the fan operates correctly.

 

 

5. Important Tips and Precautions

  • Verify correct temperature rating (e.g., ON at 95°C, OFF at 88°C)
  • Use a torque wrench to avoid cracking aluminum components
  • Always bleed air from the cooling system after refilling coolant
  • Inspect connectors for corrosion or debris before reconnecting

 

 

Conclusion

The thermistor switch may be small, but it plays a crucial role in regulating the radiator fan and protecting the engine from overheating. Fortunately, this part is often inexpensive and can be replaced with basic tools and a little know-how. With the right preparation and safety steps, you can restore proper fan function and gain confidence as a DIY car owner.