VW Polo TDI: Cooling System Mystery Causing Leaks

As someone with moderate repair experience, I dealt with a similar cooling system issue on my 2009 VW Golf TDI that showed comparable symptoms to yours. The main problem turned out to be a wiring issue between the coolant temperature sensor and the engine control module. The communication fault caused incorrect temperature readings, making the system behave erratically. Found coolant puddles just like you describe. Took it to a workshop where they diagnosed a connectivity issue in the CAN bus system. Specifically, the wiring harness connector to the coolant temperature sensor was corroded, causing intermittent signal problems. The repair involved replacing the damaged connector and cleaning the connection points, costing 95€ in total. The symptoms disappeared completely after fixing the control module communication problem. No more overheating, no check engine light, and the cooling system pressure returned to normal. To provide more specific advice for your case, could you share: Have you noticed any correlation between the overheating and specific driving conditions? What exact diagnostic trouble codes are you getting? Did you have any recent work done on the cooling system? Does the temperature gauge show consistent readings or does it fluctuate?

Thanks for the detailed response. Last service was done at 32919km including a full inspection, but these issues started recently. The overheating seems unusual and not tied to specific conditions. The data link diagnostic shows error code P0128 and P0115 consistently. Temperature gauge readings are erratic, jumping between normal and hot without warning. Had no previous cooling system work done. The control unit communication errors match what you described with your Golf. Going to have the wiring connections checked first based on your experience. The coolant sensor connector does look a bit weathered when I inspected it. Will try cleaning the connections before considering a full sensor replacement. But first want to confirm its not just a simple wiring issue causing these control module communication problems.

Based on those diagnostic trouble codes and symptoms, especially the erratic temperature readings, it definitely sounds similar to what I experienced with my VW Golf. The weathered sensor connector you noticed is a telling sign, these exact codes came up in my case too. After my repair experience, I learned these communication faults often trace back to deteriorated wiring rather than failed components. The P0115 code specifically points to a circuit malfunction in the coolant temperature sensor system, which matches the wiring issue I encountered. A simple software glitch was initially suspected in my case, but car diagnostics revealed the corrupted signals were due to that corroded connector. The coolant spillage happened because the control module was receiving incorrect temperature data, causing the cooling fan to activate at wrong times. Before replacing any parts, have the workshop perform a thorough check of all sensor connections. In my repair, simply cleaning and reseating the connector resolved the control module communication errors. If the wiring shows signs of wear, replacing just the harness section might be enough, much cheaper than swapping the entire sensor or control unit. Would you mind checking if your temperature fluctuations get worse after driving through wet conditions? This could further confirm a connection issue similar to what caused my problems.

Thanks for the input. Quick update after checking things further: The temperature fluctuations definitely get worse after rain or car washes, which matches what you mentioned about connection issues. The car diagnostics yesterday confirmed both error codes are still present. Took a closer look at the wiring near the coolant sensor and found visible corrosion on the connector pins. The CAN bus signals are unstable according to the scan tool, matching the communication fault pattern you described. Going to book it in next week to have the wiring connections cleaned and checked properly. Will also get them to inspect the entire harness section while they're at it. Makes sense to start with the simpler fixes before replacing expensive components. Really helpful to hear about your similar repair experience. The symptoms line up perfectly, from the control module errors to the unusual overheating and coolant pressure issues. Will update once I know if the wiring fix resolves everything.