Odd coolant temperature issue.
Since I got my S4 I have had a low coolant temperature on cold days (which I didn't notice when I bought it). On snowy days, the dial rarely moved above 70 degrees. If I left the car sitting still (i.e. with no air flow through the rad), it would go up to 90 and hold steady. Leaving me to believe the thermostat was stuck open.
I have accepted this, and have been living with it.
However, I recently discovered the diagnostic mode on the climate control panel and when I set this to 51c, which I am led to believe is engine coolant temperature, the readout starts just above ambient when I start the car, goes up slowly and after around 2 miles is 90, where it holds steady, even at 70mph (14 degrees ambient today, in rain). The dash readout didn't move from 60 until I stopped at home where it moved to 65 before I turned the engine off.
Do B5s have 2 different sensors for coolant, one in the engine and one in the climate control heat exchanger, or does the climate control get this temp from the ECU (like it receives revs and vehicle speed) and I have a buggy dash? It may be worth noting that the DIS is also temperamental, not in the usual missing pixels way, sometimes it's just faded in patches and doesn't really light up.