Radiator Fan Comes on When Engine is Cold – Causes and Solution

The radiator fan, a crucial component of a vehicle’s cooling system, is designed to ensure that the engine maintains an optimal operating temperature. Surprisingly, some people notice their fan activating even when the engine is cold, a scenario seemingly at odds with the fan’s primary function. This apparent contradiction can be perplexing. Why would a component primarily responsible for cooling, come into play before the engine has even had the chance to warm up? This article delves into why radiator fan comes on when engine is cold, shedding light on the intricacies of modern automotive systems and fixing the issue.

cooling fan comes on when car is cold

The Radiator Fan’s Primary Role

To better understand the anomaly, it’s essential first to establish what the radiator fan typically does. This fan is designed to increase airflow across the radiator of the engine cooling system. This process ensures that the engine maintains an optimal temperature, especially when the car isn’t moving swiftly enough for ambient air to offer sufficient cooling.

Standard Operating Conditions

Usually, the radiator fan activates when the engine’s temperature reaches a certain threshold. This is often when the engine is warm or undergoing considerable stress. But as some drivers have noticed, there are times when this is different.

Radiator fan comes on when engine is cold: Reasons

When the radiator fan activates with a cold engine, it can be due to a malfunctioning coolant temperature sensor, a faulty fan relay, issues with the engine control unit (ECU), or an air-conditioning system request. The details are shared below:

1. Faulty Temperature Sensors

The engine’s temperature is continually monitored by a dedicated sensor. This device communicates the temperature data to the vehicle’s computer system (or the Engine Control Module, ECM). A malfunctioning sensor might transmit inaccurate data, leading the ECM to believe the engine is hotter than it is.

2. Malfunctioning Thermostatic Switch

While modern vehicles primarily rely on sensors and computer modules, older car models might use a thermostatic switch. This switch, influenced by the engine coolant’s temperature, can determine the fan’s activation. A defect in this switch might cause unexpected fan operation.

3. Software or ECM Glitches

Cars today are as much about software as they are about mechanical parts. Sometimes, errors or glitches in the car’s embedded software can lead to unanticipated fan behavior.

4. Activation by Air Conditioning or Defrost Settings

In a sizable number of vehicles, the mere activation of the air conditioning or defrost settings can turn on the radiator fan. This is because the A/C compressor might require the fan to maintain optimal operating conditions, or the defroster utilizes the A/C system to reduce humidity, necessitating the fan.

Implications of Premature Fan Activation

1. Wear and Tear

The constant operation can stress the fan motor, possibly leading to its premature failure, which could affect the engine’s temperature management.

2. Efficiency Concerns

A continuously operating fan draws power, leading to a marginal yet significant decrease in fuel efficiency over time.

3. Concealing Genuine Issues

A perpetually running fan can camouflage real overheating due to a clogged radiator, potentially causing drivers to overlook them until they escalate.

How to fix the cooling fan comes on when car is cold?

When a cooling fan activates even when the car is cold, it may indicate an issue that requires addressing. Here’s a step-by-step guide to diagnose and possibly fix the problem:

1. Preliminary Checks

  • Observe the Behavior: Before diving into the technical aspects, observe when the fan turns on. Does it consistently activate with specific functions like the air conditioning or defroster? Some cars are designed to start the fan with these functions.
  • Check the Manual: Vehicle manuals sometimes contain troubleshooting steps for common issues, including unexpected fan operations.

2. Inspect the Coolant Temperature Sensor

The engine coolant temperature (ECT) sensor monitors the engine’s temperature and sends this data to the car’s computer.

  • Location: Typically found near the thermostat housing, but refer to your vehicle’s service manual for the precise location.
  • Examination: The sensor may need replacement if it has visible damage or corrosion.
  • Test: Using a multimeter, you can test the resistance of the sensor. As the engine warms up, the resistance should change predictably. If it doesn’t, the sensor might be faulty.

3. Check the Thermostatic Switch (For older cars)

Older vehicles might use a thermostatic switch instead of, or in addition to, sensors.

  • Location: Usually found on the radiator or near the thermostat housing.
  • Test: You can bridge the switch terminals with a jumper wire. The switch may be at fault if the fan turns off when bridged.

4. Examine the Relays and Fuses

Relays control high-power devices, like the cooling fan, using low power signals.

  • Location: Typically found in the fuse box.
  • Inspection: Look for any damaged or burnt fuses and relays.
  • Replacement: If you find a damaged relay or fuse, replace it. Ensure that the new relay or fuse matches the specifications of the old one.

5. Scan for Diagnostic Trouble Codes (DTCs)

A diagnostic tool can be connected to the vehicle’s On-Board Diagnostics (OBD) port. This will retrieve any error codes stored in the car’s computer, which can point towards the malfunctioning component.

6. Inspect Wiring and Connections

Check the wiring leading to the fan, the temperature sensor, and any associated relays.

  • Look for damaged, corroded, or disconnected wires.
  • Ensure that all connections are secure.

7. Software or ECM Issues

In rare cases, the engine control module (ECM) might malfunction or require an update.

  • Check for Updates: Consult with a dealership or professional mechanic if there’s a software update available for your vehicle’s ECM.
  • ECM Reset: Disconnecting the battery for several minutes can reset the ECM, potentially resolving glitches. Remember to have your radio and other electronic component codes on hand, as they might reset.

8. Consult a Mechanic

If you’ve tried the above steps and the issue persists, it’s advisable to consult a professional mechanic. They can provide a more detailed diagnosis and may identify issues that aren’t easily recognizable without specialized equipment.

When should the cooling fan come on?

The cooling fan in a vehicle serves to maintain the engine’s temperature within its optimal range. Typically, the cooling fan activates when the engine coolant exceeds a set temperature, often around 200°F (93°C), though the exact temperature can vary based on the vehicle’s make and model.

However, in some modern cars, the fan can also turn on with the air conditioning system, irrespective of the engine temperature. This ensures the air conditioner functions efficiently and prevents the engine bay from getting excessively hot. It’s also common for the fan to run for a short period after turning off the vehicle, especially after intense use, to cool the engine down.


The activation of the radiator fan during a cold start is counterintuitive. However, a blend of mechanical nuances and electronic intricacies can lead to such occurrences. By understanding the potential causes and implications, drivers can ensure vehicles operate efficiently and safely. When anomalies persist, always consider a professional’s advice.


Mr. Shafiqule Islam is a graduated Mechanical Engineer and has more than 15 years experience of repairing and maintenance of different brand vehicles like Toyota, Mitsubishi, Ford, Mercedes, BMW etc. He is also giving training to Mechanics. He has started writing to share his practical knowledge to Vehicle Owners, Drivers and Mechanics to keep their cars at best fit.

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