Coolant temp drop
 

89 vert auto

I never noticed before today ( maybe because it was 9 degrees today ) but when I was coasting down hill ( here in Hamilton,On we have a lot of accesses to the upper section of the city) the temperature guage dropped severely. It went from fully operating temperature to almost cold start temp at the bottom of the guage.

Now I know it's not combusting when I'm coasting but I have never noticed it this severe. Maybe a little bit on my last 7s but I go from half to 0 now.

I checked the level and its fine. Maybe bad sensor ? Is this a problem ? I know s5 gauges aren't the best but it doesn't fluctuate while driving on flat surfaces.

Edit: forgot to add, would blasting the heat cause a severe drop while coasting ?

It's generous to describe the S5 temp gauge as "not the best" because it's main purpose is as a warning to tell you when to buy a new engine.
Basically, the needle rises to "normal" (halfway in a S5) at about 130°F and doesn't move again till @ 220-230°F, so really, any normal temp variations (like turning on the heat) won't even register on the gauge.

Unless you recently did some cooling system work and might reasonably suspect remaining trapped air, your most likely culprit is the sending unit.

Edit:
It'd also be logical to think of the thermostat...they can cause spikes in temp as they start to fail.

Many find that descending a long grade in extremely cold weather, especially with a "small" engine,
there will be an adverse effect on the engine temp, as the extremely cold air flowing around the
engine will draw the temp down as the engine isn't generating full combustion to create heat.
Try a shade in front of the radiator.