What Causes the Car Fan to Keep Running When the Air Conditioner is On?

[DelLuke]

Water temperature sensor issues, thermostat problems, high engine temperature, or lack of antifreeze can cause this. Car fans are generally used to cool the radiator. Depending on the model, economy cars usually have one large fan, while luxury models typically feature two smaller fans. In luxury cars, the fans cool the radiator, while the air conditioning condenser is placed in front of the radiator. Here are some solutions for a non-functioning car fan: 1. Solution One: Check if the water temperature relay is faulty—replace it if there's poor contact or no response. 2. Solution Two: A faulty temperature control switch may prevent the fan from receiving power when the engine is hot. 3. Solution Three: The fan itself may be damaged due to overload or short-circuiting—simply replace it with a new one. 4. Solution Four: Wiring issues, such as short circuits or breaks, can cause the fan to stop working. Carefully inspect the wiring, as temperature regulation is crucial for the engine.

[LaMary]

That day my old Passat had the same issue, the fan was as noisy as a helicopter. After checking for a long time, I found the thermostat was stuck, causing the coolant temperature to falsely rise and triggering the fan for cooling. Additionally, the radiator fins were clogged with a thick layer of poplar fluff, and high refrigerant pressure could also make the fan spin wildly. Once, stuck in traffic on the 4th Ring Road, I even discovered the electric fan relay was stuck, and it kept buzzing after turning off the engine. I suggest first feeling the temperature difference between the upper and lower radiator hoses—if it exceeds 15 degrees, the thermostat might be broken. If the condenser is so dirty it’s turning black, spending 50 bucks to get it cleaned at the auto parts market might solve the problem.

[StAlana]

Mini Cooper owners dread this in summer! A mechanic taught me a few DIY checks: When starting the car with AC on, observe if the fan switches between high and low speeds - if not, check the fan resistor; If it keeps running after turning off the engine, inspect the AC pressure sensor for false alarms; For older cars, pay special attention to wiring damage from rodent chewing. Last time my issue was oxidized pressure switch pins - fixed by sanding them lightly after removal.

[DelLeonardo]

From the perspective of the refrigeration cycle, the constant operation of the fan essentially indicates excessive system pressure. There are four main reasons: 1. Poor heat dissipation from the condenser (blocked by willow catkins or insect remains); 2. Excessive refrigerant causing high-pressure exceedance; 3. Damage to the cooling fan control module; 4. Failure of the coolant temperature sensor. It is recommended to use an OBD scanner to read the air conditioning pipeline pressure value—if it exceeds 15 bar, the system must be shut down for inspection and repair. There was once a modified car that experienced abnormal compressor wear due to the wrong type of refrigeration oil being used, resulting in the high-pressure pipe becoming so hot it could fry an egg.

[AngelaLee]

In the workshop, when encountering this situation, we first measure the high/low pressure of the air conditioning system. Under normal conditions, the high-pressure pipe should feel hot to the touch but can be held for 3 seconds, while the low-pressure pipe should be frosted but not frozen. If the high pressure exceeds 18 kg/cm² (normal range is 12-16), it often indicates condenser blockage or insufficient cooling fan speed. Last month, when repairing a Highlander, it turned out the fan brushes were completely worn out. Replacing the entire assembly would cost 900, but actually replacing just the brushes alone only costs 30.

[VanAlexis]

Just like how a fever needs sweating to cool down, the air conditioning system relies on fans for heat dissipation. Common causes include dirty radiators (accounting for 70% of failures), overcharged refrigerant, and stuck radiator fans. Here's a practical tip: Shine a flashlight on the condenser at night—if the light transmittance is below 60%, it must be cleaned. Some car owners damage the fins by using high-pressure water guns, which isn't cost-effective. It's better to buy a specialized cleaning agent and scrub gently. Remember, the fan running for a few minutes after turning off the engine is normal pressure release, but if it continues for half an hour, there's a problem.