VW Lupo Cooling Crisis: CAN Bus Chaos Causes Overheating

Had nearly identical issues with a 2002 VW Polo diesel. Experience level: some hands-on work with VAG vehicles, especially electrical systems. The symptoms match what I encountered, rapid overheating and coolant loss. Initially thought it was just a leak, but diagnostic scanning revealed data link communication faults between the engine control module and cooling system components. The root cause was a damaged wiring harness near the coolant temperature sensor. The CAN bus wiring had partially corroded, causing communication fault codes and incorrect temperature readings. This made the system behave erratically, leading to cooling system pressure issues. Took it to a specialist who identified the exact location of the wiring issue using proper diagnostic equipment. Total repair cost was 95€, which covered the harness repair and clearing the diagnostic trouble codes. To help diagnose your case more precisely, could you share: Current mileage, Any recent repairs or maintenance, Specific diagnostic trouble codes showing, Whether the issue occurs from cold start or only after driving, If the temperature gauge shows steady readings or fluctuates This information would help determine if your Lupo has the same underlying cause as my Polo did.

Thanks for sharing your experience. My Lupo had the last service about 2 months ago with no issues found. Current mileage is 114940km. The temperature fluctuations happen mostly after 10-15 minutes of driving. The gauge readings are erratic, jumping between normal and hot within minutes. Ran a car diagnostics scan which showed multiple control module errors: P0128 (coolant thermostat) and P0116 (engine coolant temperature circuit range/performance). The communication fault seems to get worse when the engine is hot. The data link between modules becomes increasingly unstable according to live monitoring. Based on your experience, I will have the wiring harness thoroughly checked, especially around the temperature sensor area. Would you recommend any specific tests to confirm if the wiring is the culprit before heading to the shop?

Thanks for the additional details. The error codes and symptoms you describe are remarkably similar to what my Polo showed. Those exact same DTCs appeared before I discovered the wiring issue. A simple test I used before going to the shop: check the temperature sensor connector for signs of corrosion or damage, carefully wiggle the wiring while the engine runs to see if readings fluctuate more than usual. This helped pinpoint the wiring issue location in my case. The timing of your symptoms, appearing after warm-up, matches my experience. The control unit communication problems became more noticeable as heat built up, likely due to thermal expansion affecting the damaged wiring sections. Worth noting: after fixing the wiring issue, I also discovered minor insulation wear on nearby cables. The repair shop suggested replacing the entire sensor wiring section rather than just patching the obvious damage. This prevented future software glitches from recurring. If your diagnostic scan shows unstable sensor values while monitoring live data, theres a high chance youre dealing with the same type of wiring issue. The communication fault pattern affecting multiple control modules is a strong indicator. Just ensure the shop thoroughly tests the entire wiring harness section, not just the visible parts, to prevent a repeat repair.

Thanks for the helpful responses. After getting this detailed feedback, I feel much better prepared for my workshop visit. Just to provide closure on this: took the car in yesterday. The mechanic confirmed a damaged wiring harness near the coolant sensor was causing communication faults between control modules. The CAN bus signals were getting corrupted due to moisture infiltration in that area. The repair involved replacing the affected wiring section and updating several control module parameters. Total cost came to 180€ including diagnostic work and parts. The temperature now reads consistently and the cooling system maintains proper pressure. For anyone facing similar issues: the key diagnostic trouble codes to watch for are P0128 and P0116, especially if they appear together with erratic temperature readings. A proper car diagnostics scan showing communication errors between modules can help identify if its an electrical rather than mechanical problem. Really appreciate the guidance that helped me understand the likely cause before the repair. Saved time and potentially money by being able to point the mechanic in the right direction.