* Engine Coolant Temperature (ECT) Sensor: This sensor tells the car's computer (PCM or ECM) how hot the engine is. If it's faulty and not reporting accurately, the computer won't signal the fan to turn on until the engine is dangerously hot. This is the most common cause.
* Coolant Fan Relay: This relay acts as a switch, powered by the computer, to send power to the fan motor. A faulty relay might not be closing the circuit, even when the computer sends the signal.
* Coolant Fan Motor: While less likely given the symptom (only coming on when already overheating), the fan motor itself could be failing. It might still work when it receives a high-power signal (extreme heat) but not with a normal signal.
* Wiring Issues: Check the wiring harness connecting the ECT sensor, relay, and fan motor for any broken wires, loose connections, or corrosion. A poor connection can prevent the signal from reaching the fan.
* PCM/ECM (Powertrain Control Module/Engine Control Module): In rare cases, the computer itself could be faulty and not properly controlling the fan. This is less likely than the other issues.
* Fuse: Check the fuse for the cooling fan circuit – a blown fuse will prevent the fan from working at all.
Troubleshooting Steps:
1. Check the Coolant Level: Ensure you have sufficient coolant in the radiator and overflow reservoir. Low coolant can cause overheating, and the fan might only come on as a last resort.
2. Check the Fuse: Locate the cooling fan fuse in the fuse box (usually under the hood or inside the car) and inspect it for signs of damage.
3. Test the ECT Sensor: This usually requires a multimeter to check the resistance at various temperatures. Consult a repair manual for your specific vehicle to find the sensor location and test specifications.
4. Test the Coolant Fan Relay: You can often swap the relay with a similar one (check your fuse box diagram for identification). If the problem moves to another circuit, the relay is faulty.
5. Inspect the Wiring: Carefully examine all the wiring connecting the ECT sensor, relay, and fan motor for any damage.
6. Test the Fan Motor: If you've ruled out the other components, you can directly test the fan motor by applying power to it (carefully and with the proper voltage). If it doesn't spin, the motor is bad.
If you are not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic for diagnosis and repair. Overheating can cause significant engine damage.
