What causes the Mercedes-Benz C200 fan to run continuously?

[OStella]

Mercedes-Benz C200 fan running continuously indicates that the vehicle's control computer has received a signal from the engine coolant temperature sensor indicating that the engine coolant temperature is too high, thus sending a signal to drive the electric fan to operate at high speed to cool down the engine. Possible reasons for continuous operation: If the coolant temperature is indeed high, the cooling system needs to be checked. If the coolant temperature is not high, the temperature sensor should be inspected. Fan control: The four-wire fan itself is a computer module. Firstly, the two thick wires are definitely one constant power line and one constant ground. Secondly, the two signal voltages, one is controlled by the engine's duty cycle signal voltage.

[Charles]

My Mercedes C200 had the issue of the fan running wildly last summer, and it took me a long time to figure it out. The most likely cause was that the condenser in front of the radiator was too dirty, clogged with willow catkins and dust like a quilt, leading to poor heat dissipation and the engine automatically running the fan at high speed. Another time, the electric fan controller was broken—that small box is located next to the radiator, and after replacing it, everything became quiet. Actually, a malfunctioning temperature control switch or excessive pressure in the AC high-pressure pipe can also cause this. Once, at the repair shop, I saw a mechanic using a diagnostic tool to read an AC pressure value soaring to 35 bar, while the normal level is around 15 bar. My advice? First, rinse the radiator fins with a water gun. If that doesn’t work, quickly plug in a diagnostic tool to check for trouble codes. Delaying this issue can easily drain the battery.

[OAliyah]

From a mechanical principle perspective, prolonged fan operation is mostly caused by an imbalance in the cooling system. Mercedes-Benz's system involves coordinated operation of temperature sensors, an electric water pump, and a variable-speed fan module. Common triggers include poor coolant circulation, water pump seizure, or a stuck thermostat causing falsely high coolant temperature readings. Pay special attention to the electronic thermostat - last year my car's valve got stuck in the closed position while the dashboard showed a normal 90°C reading, while parts of the engine were actually overheating. Also monitor the coolant mixture ratio, as low-quality antifreeze with excessive water content lowers the boiling point and can trigger protection mechanisms. The most troublesome issue is internal short circuits in the control module, which requires oscilloscope testing of PWM signals for accurate diagnosis.

[VanRiver]

Having run an auto repair shop for fifteen years, I've serviced hundreds of Mercedes with similar issues. First, observe the fan activation timing: continued operation after engine shutdown is normal for cooling; immediate high-speed rotation upon ignition indicates a 90% probability of electrical circuit failure. Empirically, prioritize three checkpoints: 1) Sticking fan relay—open the fuse box and press to listen for clicking sounds; 2) Cracked crankcase ventilation valve—oil vapor entering the intake system can deceive the temperature control; 3) Most commonly overlooked—incorrect wiring during xenon headlight retrofits (just handled such a case last week). The most critical concern is cylinder head gasket leakage, where exhaust gases entering the coolant passage cause false overheating readings. Immediately stop the vehicle if detecting exhaust fumes in the coolant!

[CamilleDella]

If you want to handle it yourself, try these four steps: When the engine is cold, pinch the lower radiator hose—if it's normally soft but feels hard, it indicates airlock and needs bleeding; unplug the coolant temperature sensor—if the fan stops, the sensor is faulty; use an OBD scanner to read real-time data, focusing on coolant temperature and AC pressure values; finally, check if the small black block labeled K17 in the relay box is burnt. If the fan is spinning wildly along with the compressor, it's likely a pressure sensor failure. Remember to perform a cooling system reset procedure after replacing parts, like holding the AC MAX button for ten seconds. If you're unsure, don't force it—fixing a Mercedes incorrectly might lock the ECU.

[DiJulia]

Don't think a noisy fan is just a minor nuisance—it's the last resort of your cooling system to stay alive. Common triggers fall into three categories: temperature control issues like a stuck thermostat or low coolant; false signals from sensor drift or damaged wiring; and obstructed airflow due to clogged radiators or deformed underbody panels blocking vents. Last month, a neighbor's car had its fan spinning wildly until it smoked on the highway because mice nested in the grille. One particularly Mercedes-specific failure point: seized active grille shutter motors forcing the fan to compensate continuously. Pro tip: Flush your radiator before summer hits each year, but avoid high-pressure washers—they can bend the fins.