Right, im puzzled with this one now can anyone shed any light on it?

car started to turn the fans on when the radiator was cold, so I thought stat might be knackered. fitted a brand new stat which I tested before fitment. flushed all the pipes through and the radiator and refilled with new coolant (filled in the way I found on here through the top rad hose into block and rad etc). I also fitted a new header tank cap for good measure.

problem reappeared! so did some searching on here and found it might be the ecu`s water temp sender, so fitted a 2nd hand known good item (thanks apiengines).

guess what? problem reappeared!

the car isnt showing any other symptoms, oil is clear, warm air through heater, no bubbles in header or expansion tube.

is it possible that the ecu needs reseting?

Thanks in advance for any help