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.
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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.
Parts & Tools
Enter your vehicle on the home page to get vehicle-specific parts links.
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Related Issues
Bad Cooling Fan Relay
A bad cooling fan relay can stop the radiator fan from turning on when the engine gets hot. This can cause overheating at idle, overheating in traffic, weak AC performance at low speeds, or a cooling fan that only works sometimes. The relay should be tested before replacing the fan motor because a fan motor can look dead when the relay is not sending power.
Blown Cooling Fan Fuse
Blown Cooling Fan Fuse means the fuse protecting the radiator fan circuit has opened, usually because the fan circuit drew too much current or shorted The repair should start with power, ground, fuse, connector, and load testing instead of guessing at modules or replacing parts at random.
Cooling Fan Control Module Fault
Cooling Fan Control Module Fault means the electronic module controlling fan speed or fan command is not switching the fan correctly It can cause overheating, fan warnings, AC performance problems, or intermittent fan operation depending on the vehicle.
Other Cooling Issues
Browse more diagnostic guides in this category.
AC Compressor
Your ac compressor is the heart of your air conditioning system, pressurizing refrigerant to cool cabin air. A failing compressor will leave you blowing hot air and typically requires replacement rather than repair.
AC Compressor Car
An ac compressor car component is the heart of your air conditioning system, pumping refrigerant to cool cabin air. When it fails, your AC stops working entirely, leaving you in uncomfortable heat.
AC Compressor Cost
AC compressor cost typically ranges from $500 to $1,500 at a repair shop, including parts and labor, with DIY replacements costing $100–$400 in parts alone. A failing compressor reduces cooling performance and can damage other AC components if left unaddressed.
AC Compressor Oil
AC compressor oil is a specialized lubricant that reduces friction inside your compressor and keeps it running efficiently. Low or contaminated compressor oil can damage your AC system and lead to expensive repairs.
AC Compressor Price
AC compressor price varies widely based on your vehicle make and model, with costs typically ranging from $200 to $1,500 or more for parts alone. Professional installation adds labor charges, making total replacement costs significantly higher—often $500–$2,500 at a shop.
AC Compressor Pulley
The ac compressor pulley is a spinning component that engages and disengages the compressor from the serpentine belt to control when your air conditioning runs. When it fails, your AC won't work properly and you'll hear grinding or squealing noises from the engine bay.
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.