Cause
Faulty fan speed feedback signal
The signal reporting how fast the fan is actually turning, faulty so the module cannot verify what its own command achieved. The fan may be running perfectly — this is a measurement fault, and it is separated from a fan or module fault because the response to it is disproportionate and surprising.
A module that cannot confirm fan speed usually defaults to full speed. That is the safe choice: it cannot know the engine is being cooled, so it assumes the worst. A fan running flat out from a cold start, loudly and continuously, is very often this fault rather than an overheating engine — and the temperature gauge sitting at normal is the clue that it is the feedback and not the heat.
The signal is a low-current sense line and fails accordingly. A chafed wire where the loom crosses the shroud, a corroded pin in a connector sitting in the airflow at the front of the vehicle, or a failed sensing element in the fan itself. Compare commanded speed against reported speed in live data — a command that changes with a report that does not is the whole diagnosis, and it distinguishes this cleanly from a fan that genuinely cannot reach its speed.
Typically a moderate job.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
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