Fan temperature sensor

Hi after arriving home from a trip at the weekend my engine fan was on. No problem I thaught as we had been sat idleing for a while. So we went inside and had tea (20mins) and the fan was still on so I went out and disconnect the battery. I believe the fan temperature sensors on the pergeot boxers are troublesome they look easy enough to replace.
Has anyone had to replace there's?
Is it a difficult job?
Do o need to drop the coolant if so will it need replacing?
Thanks.

Tooter
1 release coolant pressure cap to release pressure then refit
2 now you can remove the old switch with out draining all the coolant have the new switch ready put your finger over the hole after removing old switch now check the new switch is the same and the seal . now you can fit the new switch with a small leakage of coolant
3 if the coolant is a red or pink in colour this is long life so will not have to be replaced
if its blue or green in colour I would replace with long life

Excellent advice glyne lock, I will try to remember that for when it's my turn.

the only safety bit I should have said was to only do this with a cold engine.
to try and drain and catch the coolant is harder then as I said above .nice to be appreciated
Gassygassy