Why does the car fan keep running all night after the engine is turned off?

[Kirk]

If the car fan keeps running all night after the engine is turned off, it may be due to a faulty coolant temperature sensor or a malfunctioning fan relay, which requires replacement at a repair shop. Water-cooled engines rely on the continuous circulation of coolant to dissipate excess heat. The coolant follows two circulation paths within the engine: a major cycle and a minor cycle. When the engine is first started and has not yet reached its normal operating temperature, the coolant undergoes the minor cycle, bypassing the radiator to facilitate rapid engine warm-up. Once the engine reaches its normal operating temperature, the coolant switches to the major cycle, passing through the radiator for cooling. An electric fan located behind the radiator draws air inward when operating, increasing airflow speed across the radiator to enhance cooling efficiency. After long-distance or high-speed driving, the engine coolant temperature remains elevated, and it is normal for the cooling fan to continue running for a while after the engine is turned off. However, if the fan relay or coolant temperature sensor is faulty, the fan may run continuously.