A 2020 Mercedes-Benz S-Class (W222) recently rolled in with the check engine light on and the cooling fan screaming at maximum speed the moment the ignition was turned. The scanner pulled two communication codes, U121A and U0147, which immediately turns a standard cooling issue into a network diagnostic job. The technician noted the customer's exact complaint: "kund säger att kylfläkten går hela tiden, motorlampa lyser". In our diagnostic platform, this specific combination of symptoms and codes on this chassis triggers a very specific diagnostic path.
Why the fan defaults to maximum speed
The diagnostic engine ranked a faulty cooling fan control unit as the most likely culprit at 90 percent probability with a medium severity. On the W222, the control unit is integrated directly into the fan assembly. It regulates fan speed based on commands from the engine control unit (ECU) and the climate control system. When this module fails, the system enters a failsafe mode, commanding the fan to run at absolute maximum speed to protect the engine from overheating.
Our data points to known issues with the capacitors inside this specific control unit, referencing service bulletin 2015050004A for certain W222 models, along with general water ingress. This internal failure severs communication, triggering DTC U121A. The presence of U0147 often follows, as the malfunctioning fan module introduces noise or disruption onto the data bus. The prescribed tests here are straightforward. Visually inspect the fan connector and wiring harness, verify power and ground to the fan control unit, and measure the PWM signal coming from the ECU.
Ruling out the harness and the Front SAM
If the module has power and ground, the next logical step targets the wiring, which our platform ranked at an 80 percent probability. Corrosion, broken wires, or high resistance in the harness connecting the fan module to the ECU and the Front SAM will sever the LIN or CAN bus signals. A corroded pin is a frequent cause of communication loss. To prove this out, the technician needs to run a continuity test on the signal lines between the Front SAM, the ECU, and the fan, followed by a voltage drop test on the main power cables.
At a 60 percent probability, the Front SAM itself comes into question. The Signal Acquisition and Actuation Module acts as the primary power and communication hub for the cooling system. An internal fault here, or moisture getting into the SAM, can disrupt the CAN and LIN buses. Since both the cooling fan module and Gateway B rely on this network, a failing Front SAM can simultaneously cause the U121A fan communication code and the U0147 Gateway B code. The testing path requires a visual inspection of the Front SAM for moisture, checking its voltage supply and ground, and using a diagnostic tool to interrogate the CAN and LIN bus status directly.
The high severity outlier
Ranked last at 30 percent, but flagged with high severity, is a failure of the engine control unit itself. It is rare, but the ECU can fail to transmit the correct pulse-width modulated (PWM) control signal to the fan module. Without a valid PWM signal, the fan defaults to failsafe mode and logs the communication error.
Because replacing or repairing a W222 ECU is an expensive and complex operation, this remains at the bottom of the diagnostic funnel. You only condemn the ECU after completely ruling out the fan module, the harness, and the Front SAM. Confirming this requires measuring the PWM signal directly at the output pin of the ECU and verifying the software calibration.
Next steps in the bay
When a W222 comes in with a screaming fan and U-codes, do not just order a fan assembly. Check the PWM signal and verify the bus integrity first. If you are seeing similar patterns with W222 cooling fans in your own shop, I would be interested to hear how often you find moisture in the SAM versus a dead module capacitor.