* Engine Cool-Down Cycle: After you turn off the engine, the coolant is still very hot. The fan continues to run to help dissipate this heat and prevent overheating. This is controlled by a thermal switch or the PCM (Powertrain Control Module) which monitors coolant temperature. It will run for a variable amount of time depending on how hot the engine was when shut off.
* Transmission Cooling: In many vehicles, including some Impalas, the transmission fluid is cooled by running it through the radiator. The fan assists in cooling the transmission fluid as well, preventing damage from excessive heat.
* AC Condenser Cooling: If you recently used the air conditioning, the condenser will also be hot. The fan helps cool down the condenser and prevent damage.
When it's NOT normal:
While continued fan operation after shut-off is usually normal, it becomes a problem if:
* It runs for an excessively long time (15-20 minutes or more): This could indicate a problem with the coolant temperature sensor, the fan control module, or a wiring issue.
* It runs constantly, even after the engine has cooled down: This points to a malfunction in the fan control system.
* The fan runs at high speed constantly: This also suggests a control system malfunction.
Troubleshooting:
If you suspect a problem, consider:
1. Check your coolant level: Low coolant can lead to overheating and prolonged fan operation.
2. Listen to the fan: Does it seem to be running at a normal speed, or is it unusually loud or fast?
3. Feel the radiator and hoses: Are they excessively hot even after a significant time?
4. Have a mechanic check the coolant temperature sensor, fan relay, and the fan control module itself. A diagnostic scan tool can often detect problems within the cooling system's electrical components.
In short, a brief period of fan operation after shut-off is expected. Persistent or excessively long operation warrants investigation.
