Fitting a genuine VAG CTS is a must. Cheap aftermarket temp sensors rarely work properly. Make sure you bleed the coolant system properly. i.e Header tank cap off, heater on hottest setting, bring the engine up to normal operating temp with the rpm's held at about 3000& hold it there for at least 5 mins. Turn off top up & leave the cap off then allow the car to cool down then top up again if necessary.