Audi Q3 Coolant Loss Mystery, No Visible Leaks Found

Had a similar issue with my 2012 Audi Q5 2.0 TFSI last summer, and I have some experience working on VAG engines. The symptoms match what I encountered, check engine light and low coolant without visible leaks. Initially thought it was the water pump too, but it turned out to be much simpler. The mechanic found a faulty connector on the coolant temperature sensor that was causing erratic readings and triggering the check engine light. Fixed it by replacing the wiring connector and cleaning the sensor contacts. Total repair cost was 95 Euro including diagnostic time. After the fix, they also did a proper cooling system bleed to remove any air pockets that might have formed in the coolant reservoir. Before jumping to conclusions about head gaskets, might be worth checking these basics: Is the coolant level dropping consistently? Does the cooling fan operate normally? Have you checked the radiator cap for proper sealing? Could you share more details about: When did you first notice the coolant loss? Any overheating warnings? What error code is stored? Does the engine temperature gauge show normal readings? This would help narrow down if your issue matches what I experienced or if its something different.

Thanks for the detailed response. Last service was done at 172834 KM and they actually checked the cooling system but found nothing concerning. Since your experience seems similar, I checked the coolant reservoir again and noticed small bubbles when the engine is running, which might indicate air pockets in the system. The cooling fan kicks in normally, and the temperature gauge stays steady. The error code is P2181 and coolant loss has been gradual over the past month. No overheating warnings so far. Will have the coolant temperature sensor connections checked first before investigating the water pump or anything more serious. Hopefully its just a sensor issue like in your case.

Thanks for the update. The bubbles you mentioned in the coolant reservoir definitely caught my attention, as I had overlooked this symptom in my initial diagnosis of my Q5. While my issue was resolved with the sensor connector, bubbles in the coolant can indicate a more serious problem. After fixing my sensor, I actually experienced this same symptom a few months later. It turned out that air was getting pulled into the cooling system through a deteriorating head gasket, much pricier fix than my initial sensor problem. The P2181 code combined with bubbles suggests possible combustion gases entering the cooling system. A good mechanic can perform a combustion leak test to confirm if exhaust gases are entering the coolant circuit. My eventual repair costs: Head gasket replacement: 1200 Euro, New coolant and system flush: 150 Euro, Various seals and gaskets: 200 Euro Before going that route, have them: 1. Pressure test the cooling system 2. Check for exhaust gases in the coolant reservoir 3. Verify the radiator hoses arent collapsing under vacuum The consistent temperature readings are good news, but dont wait too long to get it checked. My repair costs would have been lower if I had addressed the bubbles sooner.

Will definitely get the combustion leak test done first thing next week. Your experience helps a lot, especially knowing the price range for potential repairs. Really hoping its not the head gasket, but those bubbles in the coolant reservoir have me worried now. The water pump and radiator cap are also on my list to check, but given the symptoms, I understand why you went straight to checking for exhaust gases in the cooling system. Makes total sense considering the P2181 code and gradual coolant loss. I called my regular mechanic and booked it in for Monday. Will make sure they do the pressure test and check for combustion gases in the coolant before replacing any parts. Knowing it cost you 1550 Euro total helps me prepare for the worst case scenario. Thanks for breaking down the repair costs, very helpful for budgeting.