Google 'Inline thermostat' for the Audi A4 B8 2.0tdi. Its at the bulkhead end of the engine inside one of the coolant hose (lower). A pain to get to as it's buried underneath other pipes and hoses but can be reached. Once replaced temp gauge will reach 90°c and stay there guaranteed, just done mine after changing thermostat, coolant etc. Hope this helps someone having issues with engine not reaching the correct temp especially in winter.