* Faulty Fan Relay: The relay is an electrical switch that controls the power to the fans. A bad relay will prevent the fans from engaging, even when the engine temperature sensor signals overheating. This is a very common cause.
* Bad Temperature Sensor (ECT): The engine coolant temperature (ECT) sensor tells the computer (PCM) how hot the engine is. If the sensor is faulty and reading a lower temperature than reality, the PCM won't signal the fans to turn on until the engine is significantly hotter.
* Faulty PCM (Powertrain Control Module): The PCM receives the signal from the ECT sensor and controls the fans. A problem with the PCM itself can prevent it from sending the signal to the fans. This is less common than relay or sensor issues but possible.
* Wiring Problems: A break or short in the wiring between the ECT sensor, the relay, the fans, and the PCM can interrupt the circuit and prevent the fans from operating. This could be a corroded connector or damaged wire.
* Low Coolant: While seemingly obvious, insufficient coolant means the engine will overheat faster. The fans will likely *eventually* turn on, but only after the engine temperature climbs significantly higher than normal. Check your coolant level!
* Faulty Fan Motor(s): The fan motor(s) themselves could be failing. They may still work, but only when the engine is extremely hot, or they might be completely dead. You can often test this directly with 12V from a battery (with appropriate caution!).
Troubleshooting Steps:
1. Check Coolant Level: This is the easiest and first thing to do. Low coolant is a major contributor to overheating.
2. Check the Fans Directly (Safety First!): With the engine OFF and COOL, try powering the fans directly from the battery using jumper cables. If the fans run, the problem is likely in the electrical circuit leading to them. Be extremely careful doing this; it's easy to get hurt.
3. Inspect the Relay: Locate the radiator fan relay (your owner's manual or online resources can help you find its location). You can test the relay with a multimeter or simply swap it with a known good relay of the same type (from another circuit, if possible).
4. Test the ECT Sensor: This requires a multimeter and some technical knowledge. You need to check the resistance readings of the sensor at different temperatures. Online resources for your specific vehicle should provide the correct resistance values.
5. Check Wiring: Visually inspect all wiring related to the fans, relay, and ECT sensor for any obvious damage, corrosion, or loose connections.
If you're not comfortable with electrical testing, it's best to take your vehicle to a qualified mechanic for diagnosis and repair. Overheating can cause significant engine damage.
