low temp thermostat

[Martin]

Martin where in the cooling system do you take your coolant temperature measurements?

The top radiator hose is a good location - that's the hot water inlet into the radiator

I used an inline hose adapter like this:
(So much easier and safer than drilling & tapping into engine parts which are mostly alloy)

Wondering if VCDS looks at the engine coolant temp sensor, some feedback from the radiator fan switch or a calc of both?

Depends what type of water sensor your car has
If it's an actual temp sensor connected to the ECU then VCDS can display the temp reading
If it's a sensor that switches at a set temperature then it's just an on/off switch which will be driving a relay to turn the fan on

Switches and relays just turn electricity on/off - they don't give feedback