My Astra has a factory fitted one with a resolution of 0.5 deg
C, that too is
very accurate (measured against screen and AWS). I have no
idea where that is
mounted but it works in all weathers, a little snow symbol
even lights up when
it falls to 2 deg C which is fun in winter!
Will.

Mine beeps at me when the temperature falls to 3.5ºC. In my
previous car the display started flashing, and produced a little
snowflake symbol, when it fell below 4ºC.

At what temperature ought one to start thinking about ice on the
road? I would have thought 4ºC because that's the point at which
water starts to expand again??

Anne