Audi S3 Overheating Due to Control Module Malfunction

I had a very similar issue with my 2012 Audi S4 2.0T. With some experience working on VAG cars, I initially thought it was just a basic cooling system problem, but it turned out to be more complex. The symptoms were identical, engine overheating, coolant loss, and warning lights. After taking it to a workshop, their car diagnostics revealed a connectivity issue with the coolant temperature sensor data link to the engine control module. The sensor was sending incorrect readings, causing the cooling fans to operate erratically. The root cause was a damaged wiring harness connector near the sensor, creating intermittent signal problems. The workshop found this using detailed data link testing. What seemed like a major cooling system failure was actually just a software glitch caused by faulty wiring. The repair involved replacing the wiring connector and recalibrating the control module, costing 95 Euro in total. Since the fix, the cooling system has worked perfectly. Can you share more details about your S3: Have you scanned for specific fault codes? Does the temperature gauge show erratic readings? Are the cooling fans running constantly or not at all? Any recent work done on the cooling system? This would help determine if your issue matches what I experienced.

Thanks for the detailed response. I had the car scanned yesterday and it showed diagnostic trouble code P118A. Cooling fans are completely non-operational now, and the temperature gauge jumps erratically between normal and hot. Last service was at 174000 KM where they replaced the coolant and thermostat. Based on your experience, it does sound similar to a control module communication problem rather than just mechanical cooling system failure. Will get the wiring connections checked first before replacing more expensive components.

Definitely sounds like we have the same underlying issue. After getting my S4 fixed, I learned quite a bit about these control module problems. The P118A diagnostic trouble code you mentioned specifically points to a CAN bus communication fault affecting the engine coolant temperature circuit. The erratic temperature gauge behavior matches exactly what I experienced, it is a classic sign of corrupted sensor data not being properly transmitted through the control module network. The non-functioning cooling fans are another red flag, as the fan control module likely isnt receiving valid temperature data to trigger operation. In my case, the damaged wiring connector was causing similar communication errors. However, I would suggest having the entire CAN bus network tested, as sometimes the issue can be in other connection points between control modules. The fact that your cooling system components were recently serviced makes an electrical/communication issue even more likely than mechanical failure. When the shop does the diagnostics, make sure they: Test signal integrity across all coolant temperature sensor circuits, Check the grounds and power supply to related control modules, Verify CAN bus network resistance values, Monitor live data from both the temperature sensors and fan control signals The fix should be relatively affordable if its just wiring-related like mine was. Even if they need to replace a control module, it would still be cheaper than major cooling system repairs.

Thank you for sharing those details. After scanning and finding diagnostic trouble code P118A, plus experiencing non-operational cooling fans and erratic temperature gauge readings, I am now convinced this is a communication fault rather than mechanical failure. This matches my initial suspicion about control module issues. Your recommendation to check wiring connections first makes perfect sense, especially given my car has a similar mileage at 174000 KM and recently had cooling system work done. The symptoms really do point to CAN bus or data link problems rather than component failure. Will have the shop inspect the complete control module network and wiring harness before replacing any major cooling system parts. Will update once I know the exact cause and repair costs. Hoping its just a wiring connector issue with a reasonable 100 Euro fix like in your case rather than needing a full control module replacement. Again, thanks for confirming my suspicions about this being an electronic/connectivity issue rather than mechanical. It helps narrow down the diagnostic path considerably.