coolant sensor

hello and here's one for you
What does it mean when the temp gauge is a millimetre above middle and when i remove the sensor plug while engine is running and put it on again it shows the needle dead center.is the sensor duff?by the way i have added a ground cable from bat neg to alternator body so this excludes bad grounds.
The protective rubber of the sensor plug is cracked but it is making good contact with the sensor.and the sensor itself isn't screwed all the way down to the manifold,half a centimetre above the manifold,is urs like that also?
Please help guys,duno if i replace the sensor or the prob is with the plug itself.

bump

I will look at mine later and revert - but am sure mine is the same

i have replaced the sensor today,made a diffrence in the behaviour of the needle but still showing a very tiny bit over half and when engine switched off it goes to centre.guess this is how it is.no grounds pro at all,and no overheating.the old sensor was full of limestone.so now what?the needle is duff?

1 mm above the mid position is absolutely fine, i would not worry about this. You are correct to check for ground shifts as this is why the sensor was changed at 97MY. An easy way to check if there is a ground problem with the engine is to run engine and switch on the front/rear screens and headlamps and check if the temp gauge needle moves up. if needle moves when switching on the electrical loads you have an earthing problem. If the needle goes to the mid position with the ignition off then there is a problem with the Ipack. A lot of white paste is not good as this same paste will be in the radiator and will eventualy block it causing overheating. Caused by incorrect coolant.