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The module driving the cooling fan at a controlled speed rather than simply switching it on and off, and the part that fails far more often than the fan motor it commands. It is the reason a fan can be stuck at full speed, stuck off, or running at a speed nobody asked for, all with a healthy motor attached.

Its position is unforgiving. It is usually mounted in the airflow at the front of the vehicle, sometimes on the fan shroud itself, where it meets water, salt and stone impact while dissipating a lot of heat of its own. Corrosion at its connector and heat damage to its output stage account for most failures, and both are visible before anything is tested electrically.

The codes divide into two claims and it is worth knowing which you have. Over- and under-speed codes are the module failing to achieve a commanded speed, which can equally be a stiff motor or a sagging supply. Fan speed sensor codes are the module unable to measure what the fan is doing, and a module that cannot measure speed will often default to full speed as a safe fallback. A fan running flat out from the moment of start-up is usually that fallback, not a demand for cooling, and the engine may be stone cold.