My radiator fan (2001 Audi A4 Sport Quattro 1.8t) has started to spin as soon as I turn the ignition key. This is before I start the engine
If I sit there for a few minutes it just keeps going, sounds a bit like an aircraft taking off!

I've read that some folk have problems with it staying on after ignition switched off, but I don't seem to have this problem.

Maybe it is a faulty temp sensor? If this is a problem and not just the car 'working as designed', does anyone have some basic instructions on how to change sensor - or is a garage job?

Much appreciate your expert views