S5 Coolant Warning Mystery Despite No Visible Leaks

Had the same problem on my 2017 S4. After multiple coolant warnings, the issue turned out to be more complex than just air pockets in the system. The coolant reservoir was actually fine, but the electrical connection to the sensor was corroded and sending false readings. First step was checking all radiator hoses and the water pump for visible leaks, found nothing. The shop did a pressure test on the cooling system which confirmed no external leaks. They also tested for combustion gases in the coolant to rule out head gasket failure. The real problem was traced to a faulty wiring connection at the coolant level sensor. The connector had moisture damage causing intermittent false readings. This triggered both the coolant warning and check engine light. Key questions to ask the shop: Have they pressure tested the entire cooling system, Checked the coolant sensor wiring and connections, Verified the coolant level sensor is functioning correctly, Tested for combustion gases in the coolant While not a catastrophic issue, it required careful diagnosis to avoid replacing parts unnecessarily. Make sure they document all test results before starting repairs.

Thanks for sharing your S4 experience. My situation was similar but ended up being a failing water pump that wasnt caught during the pressure test initially. The coolant reservoir would look full but slowly drop over a few days. The radiator cap was also replaced since it wasnt holding proper pressure. Did you remember roughly what the total repair cost was? Just curious if any other issues popped up since getting it fixed. Want to avoid throwing parts at the problem like my last shop tried to do.

Just checked my records, the repair was much simpler than your water pump issue. The total came to 95 Euro to fix the corroded sensor connection. Since the repair, the engine overheating warnings completely stopped and the coolant level has stayed steady. No more dashboard alerts either. Good thing your shop caught that failing water pump, those can lead to serious engine damage if left unchecked. The faulty radiator cap was also smart to replace since it helps maintain proper system pressure. My fix has held up perfectly for the past 8 months with zero coolant leaks or sensor issues. The key was finding a shop willing to properly diagnose rather than just replacing parts. The pressure test they did was thorough enough to rule out any issues with the radiator fluid system, even though it initially missed your pump problem. Make sure to keep an eye on your coolant level for the next few weeks just to confirm the new water pump is working correctly. Even small leaks can lead to bigger problems if not caught early.

After seeing my last mechanic perform a sloppy diagnosis, I took the car to a new shop last week. They found the water pump was failing during a second pressure test, the first one somehow missed it. Ended up costing 780 Euro for the water pump replacement and a new radiator cap. The mechanic showed me how the old pump had small cracks causing the slow coolant leak. Been monitoring it closely for the past week. The coolant level has finally stayed constant and no more warning lights. Pretty relieved it wasnt a head gasket issue. The new shop took time to explain everything and even showed me the worn parts they replaced. Way better experience than getting the runaround from my previous mechanic who wanted to replace multiple sensors without proper testing.