Cause
Debris obstructing the fan
Something fouling the cooling fan — a plastic bag drawn against the shroud, leaves packed into the blades, a stone, or a piece of trim. The fan cannot reach its commanded speed, so the module reports an underspeed or performance fault naming the fan or its control while both are healthy.
A partially obstructed fan draws more current and runs hot, so an obstruction left in place damages the motor and its control module in turn. The noise is usually the first sign — a rattle, a scraping, or a rhythmic knock that follows fan speed rather than engine speed, which is a useful distinction when listening.
It is worth checking because it is free and immediate. Look at the fan and shroud with the vehicle off and isolated, and turn the fan by hand through a full revolution. Debris packed between the radiator and the fan is easy to miss from above, so look from below as well — and a fan blade broken by whatever went through it will be out of balance, which destroys the bearings even after the obstruction is removed.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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