Cooling fan runs constantly, even when the engine is cold

Symptoms

• Fan runs immediately on startup, even in cold weather
• Fan never seems to turn off, even after the engine is shut off for a while
• No overheating symptoms otherwise
• Possible increased road noise or reduced fuel economy from constant fan operation

Likely causes

• CommonStuck cooling fan relayA relay that's stuck closed keeps sending power to the fan regardless of actual temperature.
• CommonFaulty coolant temperature sensor reading falsely highIf the sensor tells the computer the engine is hot when it isn't, the fan runs constantly to try to cool it.
• PossibleFan control module malfunction (on vehicles with variable-speed fans)A dedicated module controlling fan speed can fail in a way that keeps it running.
• Less commonWiring short causing constant power to the fanDamaged wiring can bypass normal control and keep the fan powered continuously.

What to do

1. Check whether the fan runs even with a cold engine right after starting, which confirms it's not responding to actual temperature.
2. Check the fuse box for a cooling fan relay and try swapping it with an identical relay from another circuit to test.
3. Get any check-engine light codes read, since a faulty temperature sensor often sets a corresponding code.
4. Note if the fan continues running with the key off and engine shut down, which is normal for a short time on some vehicles but not indefinitely.
5. Have a mechanic test the temperature sensor readings against actual engine temperature to confirm if it's giving false data.
6. Replace the confirmed faulty relay, sensor, or fan control module.
7. Keep in mind that a constantly running fan, while not dangerous, does waste some fuel and adds unnecessary wear.

Tools and parts

• Fuse puller/tester
• OBD-II scanner

When to call a pro

See a mechanic if swapping the relay doesn't resolve it, since testing the temperature sensor or fan control module needs a scan tool and some diagnostic time.

Safety