I'm a bit confused on the scale of mine, as I've noticed it works fine as it should but the scale at the point of freezing goes 1, 0, -0, -1, I always thought the scale should be 1, 0, -1???? does this mean it's detection is sensitive enough to calculate if the temp range is 0 to 0.9 or 0 to -0.9? thus dependant whether it displays the minus or not on the 0 should it be less than 1 degree either way?
Our Lag II ( 2004) displays - 0 as well, so does our 2006 Megane!
I have always put it down to the fact that the sensor and electronics are capable of calculating in smaller increments but the actual display only displays whole numbers.
If your display says - 0 I assume the temp is somewhere between 0 & -1, if it shows 0 the temp is somewhere between 0 & +1.
As it is only an indication & guide, rather than the actual temp I think the resolution is adequate.
Currently in SportTourer's garage: 2010 Laguna III SportTourer Initiale Tom Tom.
Mine was reading 80 degrees always as well. Found it is located in passenger side wing mirror. Got a new one from scrap yard for pence and just wired it in. Excellent now tells me how cold it is here today!
I got new one on a virtually complete mirror - wired it an and - started at 77 but now back at 80. I believe 80 indicates a short in the wiring, but whether its between sensor and mirror, or between mirror multiblock and display I dont know - someday will try and trace.
Currently in Fiddler's garage: 2005 Megane II 1.9dCi
Might be worth checking the wiring that passes between the door and the A pillar as with the hundreds if not thousands of door opening and closings through it's life it can make these wires snap eventually.