* One fan (usually the lower one) typically runs at lower temperatures to maintain a normal operating temperature. This is often controlled by the engine coolant temperature sensor.
* Both fans run at higher temperatures or under heavy load. This provides more cooling capacity when the engine is working hard (e.g., idling in hot weather, towing, or prolonged high-speed driving). The system might also engage both fans with the air conditioning running.
If only one fan is working and the engine is overheating, there's a problem. However, it's perfectly normal for only one to run under typical driving conditions. If you're concerned about your Impala's cooling system, check your coolant level and have a mechanic inspect the system if you suspect a malfunction.
