Cause
Faulty cooling fan control module
The module driving the cooling fan at a controlled speed, failed. It fails far more often than the motor it commands, because it dissipates real heat while mounted in the airflow at the front of the vehicle, in spray and salt.
Its failures divide into stuck off and stuck on. Stuck off overheats the engine in traffic; stuck on runs the fans from a cold start, which is usually a safe default rather than a demand for cooling. Check whether the engine is actually hot before assuming the latter is a cooling fault. Corrosion at its connector and heat damage to its output stage account for most failures, and both are visible before anything is measured. A stiff or dragging fan takes the module with it, so check the fan spins freely before fitting a replacement.
Typically a moderate job.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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