citroen xsara stop light ?

Got a problem with the car that has came into the garage

When the cooling fan is about to come on the stop light comes on aswell as the overheating tempature light (although the car is just above the 90degree mark so not over heating)

our customer is a bit concerned about this and wants it sorted out

The cooling fan works and we have replaced the tempature sensor but the problem is still there?

the car is not over heating and all fluids are ok