* Faulty Fan Relay: This is the most common culprit. The relay is an electrically controlled switch that powers the fan. A stuck-closed relay will keep power flowing to the fan even after the ignition is off.
* Bad Fan Control Module (PCM or related): The Powertrain Control Module (PCM) or a dedicated fan control module might be malfunctioning. It could be sending a signal to keep the fan running even when it shouldn't. This is less common than a bad relay but more difficult to diagnose.
* Short Circuit in the Wiring: A short circuit in the wiring harness leading to the fan or the relay could provide a constant power source to the fan. This is potentially dangerous and could lead to a fire.
* Faulty Temperature Sensor: While less likely to cause the fan to run *continuously* after shutdown, a faulty coolant temperature sensor that's providing a falsely high temperature reading could trick the system into keeping the fan on. This would usually be more likely to be continuous running *while* the engine is running, but a failing sensor might cause erratic behavior.
* Low Coolant Level (Less Likely): A very low coolant level might trigger the fan to run to prevent overheating, even if the engine is off. However, this would usually only be for a short time as the system cools down.
Troubleshooting Steps:
1. Check the Fan Relay: Locate the fan relay (usually in the underhood fuse box). Try tapping it gently or swapping it with a known good relay of the same amperage and type. If the problem goes away, you've found your culprit.
2. Check the Wiring: Visually inspect the wiring harness leading to the fan and relay for any signs of damage, chafing, or shorts.
3. Coolant Level: Check your coolant level.
4. Advanced Diagnostics (If necessary): If the above steps don't solve the problem, you'll likely need to use a multimeter to test the voltage at the fan's power wire and the relay. You might also need to consult a repair manual or a professional mechanic for advanced diagnostics, possibly including scanning the PCM for trouble codes.
Safety Warning: Working on electrical systems in a car can be dangerous. Always disconnect the negative battery terminal before starting any work. If you're not comfortable working with car electrical systems, take it to a qualified mechanic. A continuously running fan can drain your battery, and a short circuit could be a fire hazard.
