Well. . mine used to come on and stay on for ages! sometimes go off and be fine! or other times come to car in the morning and totally dead! changed the sensor in the rad to the right twice! first with a used part and second with a brand new part. . also cut off the plug and re-wired with new ones. . still no luck.
turns out it was a fan control unit which is screwed under the water bottle (small black box) it had rusted etc as well as the plug so got a new one from the breakers(same part number too) for a tenner with the plug, swapped the black box, cut off old plug and wired new one up. sorted!
