Audi TT Mystery Coolant Loss Without Visible Leaks

As someone with some experience working on similar issues, I had this exact problem with my 2002 Audi A4 1.8T. The symptoms were identical, persistent low coolant warnings without visible leaks. After investigating, my workshop found that the wiring connection to the coolant level sensor was corroded and sending false readings to the ECU. There were no actual coolant leaks or engine overheating issues. The repair involved cleaning and resealing the sensor connections, which cost 95 Euro in total. Before jumping to head gasket conclusions, I would suggest checking: The radiator cap seal condition, All radiator hoses for tiny leaks, especially around clamps, The coolant reservoir for hairline cracks, The electrical connections to the coolant level sensor Could you share: Does the coolant actually drop in the reservoir when measured cold? Are there any air pockets visible in the coolant when engine is running? Have you noticed any sweet coolant smell around the engine bay? What engine variant does your TT have? This information would help determine if you are dealing with an actual leak or potentially just a sensor issue like I had.

Thanks for sharing those details about the sensor issue. In my case, I can confirm the coolant level actually drops in the reservoir when measured cold. Last check was at 41076 KM during regular maintenance. I notice small air pockets forming in the coolant reservoir while the engine is running, especially after driving for about 15 minutes. There is a faint sweet smell around the engine bay, but it is not very strong. I have tried pressure testing and found a slight leak from the plastic coolant flange near the thermostat housing. The plastic seems to have developed hairline cracks, possibly causing the coolant loss. I think this might be a more straightforward fix than a faulty sensor or head gasket issue. Has anyone replaced this part before? Would be helpful to know if this is likely the root cause before proceeding with the repair.

Thanks for the update and extra details about your TT. The symptoms you describe, especially the air pockets in the coolant reservoir and the sweet smell, match what happens with a cracked coolant flange. When I had my A4 fixed, the workshop initially focused on the sensor, but later discovered similar hairline cracks in the coolant flange. The total repair ended up costing 280 Euro, including parts and labor. The plastic becomes brittle over time due to heat cycles. The repair process involved: Draining the cooling system, Replacing the plastic flange with an updated version, Installing new radiator hoses where they connect to the flange, Refilling with fresh radiator fluid and bleeding the system After the repair, my coolant level stayed stable and the check engine light remained off. Make sure your workshop properly bleeds the cooling system to prevent air pockets from forming again. One tip: while they are working on it, have them inspect the cooling fan operation and radiator cap pressure retention. These components often need attention at similar mileage points. Let us know how the repair goes. Based on the symptoms, I am confident the flange replacement will resolve your coolant loss issues.

I appreciate all the detailed feedback about the coolant flange issue. After getting the repair done last week, I can confirm this was indeed the root cause of my coolant loss. The workshop charged 295 Euro for the complete job, including new radiator fluid and a replacement radiator cap they recommended during inspection. The cooling system now holds pressure perfectly, and the coolant level in the reservoir has remained stable. No more air pockets visible while running, and that sweet smell around the engine bay is gone. The check engine light has stayed off for over a week of regular driving. For anyone facing similar symptoms with an older TT, definitely check the plastic coolant flange near the thermostat housing first before worrying about head gasket problems. The cracking seems to be a common failure point as these cars age. The shop also confirmed the cooling fan and other components are working properly, so hopefully this fix will last. Thanks again for helping diagnose the issue and saving me from potentially unnecessary repairs.