The sensor is in the front of the resovoir, so going up an hill or parking on uneven ground can cause it, I have filled my bottle up above the fill level and then when the engine was hot checked it again and as long as it hasn't rose to the overflow pipe top it up a bit more, It would be interesting to know how often you have topped this up , I have just topped mine up the first time in 3 years. (This was after buying a new sensor £25 and realising that it wasn't the problem )
Neil
