* Faulty Fan Control Module (or Relay): This is the most likely culprit. The module/relay receives signals from various sensors (engine temperature, A/C pressure) and controls when the fan should turn on and off. If it's malfunctioning, it might send a constant "on" signal, regardless of the engine temperature. This is often a simple, relatively inexpensive fix.
* Stuck Fan Relay: Similar to the above, but instead of the entire module, just the relay itself might be jammed in the "on" position. This is an easier check than replacing the whole module.
* Shorted Wiring: A short circuit in the wiring leading to the fan or the control module can also cause it to run continuously. This requires careful inspection of the wiring harness.
* Faulty Temperature Sensor: The engine's coolant temperature sensor tells the control module when to engage the fan. If this sensor is malfunctioning or sending a false high-temperature signal, the fan will run constantly. A faulty sensor will often give a faulty reading to the gauge as well (if your gauge is working correctly).
* Low Coolant: While it might seem counterintuitive, low coolant can actually cause the fan to run continuously as the engine tries to prevent overheating. Check your coolant level first – it's the simplest and cheapest check!
* Faulty Coolant Fan Switch (if equipped): Some vehicles use a separate switch in the radiator that activates the fan based on coolant temperature. A faulty switch stuck in the "closed" position will cause the fan to run constantly.
* Problem with the A/C System: If the A/C system is malfunctioning and constantly putting a demand on the cooling system, it can cause the fan to run all the time (especially if the compressor is constantly cycling). This will likely be associated with other A/C issues as well.
Troubleshooting Steps:
1. Check Coolant Level: This is the easiest first step. Low coolant is a serious issue and needs addressing immediately.
2. Check the Fan Relay (and Fuse): Locate the fan relay in the fuse box (usually under the hood or in the passenger compartment) and try swapping it with a known good relay of the same type (if possible, check the amperage rating). Also check for a blown fuse related to the cooling fan.
3. Inspect Wiring: Visually inspect the wiring harness leading to the fan and control module for any obvious damage, fraying, or shorts.
4. Test the Coolant Temperature Sensor: This usually requires a multimeter and some knowledge of automotive electrical systems. A faulty reading should be investigated.
5. Check the Fan Control Module: If the other steps don't reveal the problem, the control module itself might be faulty and need replacement. This often requires a scan tool for verification.
If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic for diagnosis and repair. Running the fan constantly will draw a lot of power, potentially damaging the battery or other components in the long run.
