The gauge sensor is different to the fan switch. The gauge sensor is hidden on the back of the cylinder head below the crank angle sensor with a single spade terminal on it.
As Fergyuk said, the cause of the long running fan may be restricted coolant flow in the rad but something to bare in mind is it may also be caused by restricted airflow through the radiator.
I've known NB's to have this issue & I've yet find the cause. The fan just runs & runs once the engine is up to temperaure and just hold the engine temp stable, it doesn't increase or decrease. I find that if I just lift the revs up to a fast idle of around 1500rpm, this seems to circulate the coolant that little bit more and the engine temp drops and the fan stops. I think NB's just have a very low flowing cooling system.
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