Playstation 3 (PS3 CECHG04) Fan running fast and noisy

On start-up, the fan runs quietly, but after a few moments, perhaps 30 seconds, the speed (and noise) increases, and then a bit later it increases again, until it sounds like a vacuum cleaner. The exhaust air is barely warm.

This is happening while it is on the menu, without any games running.

It has been kept clear of any airflow obstructions, cleaned internally, has had new thermal paste applied and new thermal pads. The thermal paste is making good contact. It still steps up the speed.

So I'm thinking the thermal sensor(s) is/are faulty. Where are they and can they be replaced?

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