VW Beetle TDI Cooling Crisis: Leaks Under Pressure

I had a very similar issue with my 2012 VW Golf TDI last summer. As someone with some experience working on VWs, I initially thought it was just a simple coolant leak, but it turned out to be more complex. The diagnostic trouble codes from the control module were inconsistent, just like yours. After taking it to my regular workshop, they found that the cooling system control unit was getting incorrect readings due to a damaged wiring connection. The coolant temperature sensor plug had corroded, causing intermittent communication failures in the data link system. The fix involved replacing the wiring harness connector and reprogramming the control unit. Total cost was 95€ for parts and labor. Since then, no more overheating issues or warning lights. Before suggesting specific solutions, could you share: Have you noticed any correlation between the overheating and specific driving conditions? What exact warning lights are you seeing? Did you get specific error codes when scanning? Is the coolant loss gradual or sudden? These details would help identify if your issue matches what I experienced with my Golf TDI.

Thanks for the detailed response. The overheating tends to occur mainly during highway driving, especially after 20-30 minutes. The warning lights show engine temperature and coolant warnings, with occasional check engine indicators. Latest scan showed P0128 and P2181 codes suggesting thermostat/cooling performance issues. The coolant loss is fairly gradual, losing about 1L every few weeks. My latest service was just 2 months ago where they did a full cooling system pressure test that showed no obvious leaks. The connectivity issue with control modules appearing and disappearing during diagnostics makes me think your wiring harness suggestion could be spot on. Would be great to know the specific part numbers you used for the repair, since our cars seem to have very similar symptoms pointing to a wiring issue rather than mechanical problems.

The wiring harness connector I replaced was part number 1K0973733 with an additional ground wire kit 1K0971225. The symptoms you describe match my experience exactly, particularly the control module communication dropouts during highway driving. After installing these parts, my workshop found the CAN bus signals were still unstable. A software glitch in the engine control unit was causing erratic temperature readings. They had to perform a complete control module reset and reprogram using the latest firmware version. The total system pressure test revealed the thermostat was cycling erratically due to incorrect temperature signals from the damaged wiring. Once the connectivity issue was fixed and control unit reprogrammed, the cooling system pressure returned to normal and the coolant loss stopped completely. To be thorough, have your workshop check the coolant temperature sensor resistance values. In my case, the corrupted signals from the faulty wiring made the sensor readings jump between extreme values, triggering the overheating warnings despite normal actual engine temperatures. The repair sequence that worked for me: 1. Replace wiring harness connector 2. Install new ground wire kit 3. Reset engine control unit 4. Update control module firmware 5. Calibrate temperature sensors

The diagnostics details and repair steps make a lot of sense, especially given how similar our symptoms are. Looks like I need to focus on the electrical side rather than just the mechanical cooling components. Will get those specific part numbers you mentioned, the 1K0973733 harness connector and 1K0971225 ground wire kit. Having these exact parts will save a lot of time at the workshop. The complete repair sequence you outlined matches what my mechanic suspected might be needed, particularly the control module reset and firmware update steps. My CAN bus communication issues during highway driving really seem to mirror what you experienced. Good point about checking the temperature sensor resistance values too. The fact that your coolant loss stopped completely after fixing the connectivity issue and reprogramming the control unit is encouraging. Will book the car in next week and have them follow this repair path. The corrupted sensor readings and pressure issues you described explain why my cooling system seems mechanically sound but still acts up. Thanks for the comprehensive diagnostic trouble code based solution rather than just suggesting parts replacement.