VW Phaeton Coolant Loss Mystery Without Visible Leaks

I had a similar issue with my 2014 VW Passat. With some experience working on VWs, I can share what happened in my case. The symptoms were identical, check engine light, low coolant level, but no visible coolant leak under the car or in the engine bay. After checking all the obvious spots like radiator hoses and connections, I took it to my regular workshop. The diagnostic revealed a faulty wiring connection to the coolant level sensor, which was causing incorrect readings and triggering the check engine light. The repair involved replacing the damaged wire harness and connector. Total cost was 95€ including parts and labor. However, your situation might be different. To provide more specific advice, could you share: Are you noticing any white exhaust smoke? Does the engine temperature gauge show normal readings? Have you noticed any coolant puddles after parking? How much coolant are you losing and how often do you need to top it up? These details would help determine if youre dealing with a simple sensor issue like mine or if there might be actual engine overheating or head gasket concerns that need attention. Always worth having air pockets checked in the cooling system too, as they can cause false readings. Make sure the radiator cap is tested during diagnosis, as a failing cap can cause pressure loss and coolant level issues.

Thanks for the responses so far. Just to update: I checked the car this morning and noticed white smoke from the exhaust during cold start. The coolant loss seems quite steady, about 1/4 of the coolant reservoir every 2 weeks. Engine temperature stays normal during my commute. Last service was done 3 months ago at 22867km where they changed oil and filters. They didnt mention any cooling system issues back then. The water pump seems to be working fine, but the smoke and consistent coolant loss point toward possible head gasket failure. I called two workshops and waiting for appointments. Still welcome more experiences, especially regarding repair costs for similar issues on VW engines.

Thanks for the update. Having owned a 2014 VW Passat myself and faced cooling system issues, the white smoke combined with steady coolant loss definitely rings alarm bells. My initial thought about the sensor connection was too optimistic. The symptoms you describe, especially white smoke on cold start and consistent coolant loss, strongly suggest internal coolant leakage. While my issue was resolved with simple wiring, your case shows classic head gasket failure indicators. When my water pump failed last year, it showed different symptoms, more sporadic coolant loss and engine overheating warnings. Your steady coolant consumption without temperature fluctuations points to coolant entering the combustion chamber. The repair cost on my Passat for a head gasket replacement was 1200€, including pressure testing and machining the head surface. Labor took about 12 hours. Make sure the workshop checks the entire cooling system during repair, radiator cap, radiator hoses, and water pump, since they need to drain the system anyway. Get it fixed soon to prevent engine overheating and more extensive damage. Consider requesting a cooling system pressure test to confirm the diagnosis before major work begins.

Thanks for the input. After reading these responses, I decided to take action today. Called a third workshop that specializes in VW engines and managed to get an appointment for next week. The symptoms match what others described for head gasket issues, the white smoke during cold start and steady coolant loss without temperature problems are telling signs. Before the appointment, I will monitor the coolant level daily and check the radiator hoses again just to be safe. Really helpful to know the actual repair costs shared here. Will budget around 1200-1500€ for the worst case scenario of head gasket replacement. At least now I know what questions to ask at the workshop and what to expect during diagnosis. For now, I will avoid any long trips and keep an eye on the temperature gauge. Will also get them to check the radiator cap and look for air pockets in the system during the repair, as suggested. Will update this thread once I get the final diagnosis and actual repair cost for future reference.