Cooling Fan Runs Constantly
A cooling fan that runs constantly can slow warm-up, reduce fuel economy, or point to sensor, relay, AC pressure, or module command problems.
Can I Drive?
monitor
Most Likely Causes
- 1
Stuck fan relay
A relay with stuck contacts can power the fan even when not commanded.
- 2
Coolant sensor reading too hot
False high temperature data can keep the fan running.
- 3
AC pressure request
High AC pressure or a pressure sensor fault can command fan operation.
- 4
Fail-safe strategy after sensor fault
Some vehicles run the fan constantly when temperature data is missing.
How to Diagnose It
- 1
Check commanded fan status
Use scan data to see whether the module is commanding the fan on.
Tool: Scan tool
- 2
Swap or test relay
Check whether the fan stops when the relay is removed or replaced with a correct relay.
Tool: Fuse box diagram
- 3
Compare coolant temperature reading cold
A sensor reading hot on a cold engine is suspect.
Tool: Scan tool
How to Fix It
Replace stuck relay if confirmed
Use the correct relay rating and pin layout.
Repair coolant temperature sensor circuit
Fix the sensor or wiring causing false high readings.
Diagnose AC pressure input
If fan command follows AC pressure data, test the AC sensor and refrigerant pressures.
Mistakes to Avoid
- Do not remove the fan relay as a permanent fix.
- Do not ignore a fan that runs after key-off until the battery dies.
- Do not replace the fan motor when the control side is commanding it on.