Hello, I have determined the following: My Phaeton is only warm at cold outside temperatures (under 7…8°C) very slowly. 90°C coolant temperature is reached after approx. 30km of highway. If it is warmer the problem does not exist. The temperature is then reached in (feeling) appropriate time. Exactly this behavior would be observed if the heater was defective, however, this is not the case as far as I can judge. The error memory does not contain any entries. If the thermostat was defective, I would assume that the 90°C would never be reached at all? How should you do the best or where could the error lie? On suspicion of replacing the thermostat? Or also the thermostats.. I remember darkly that there are two pieces in the 3.0 TDI? The problem could last longer. I hardly notice this, since I drive almost only in the summer. Also, because of the lack of cold temperatures I could then not to determine at all whether the thermostat change has brought something. MfG