VW Golf TDI Cooling System Failure Points to ECU Issue

I experienced similar symptoms with my 2015 VW Passat TDI. Being somewhat experienced with car maintenance, I initially thought it was just a coolant leak, but the control module communication errors pointed to something more complex. The data link connectivity issue between control modules was causing the cooling system to malfunction. The workshop discovered a damaged wiring harness connector that was disrupting the signals between the engine control unit and cooling system control module. The repair involved replacing the faulty connector and recalibrating the control modules. Total cost was 95€ for parts and labor. Since the fix, no more coolant leaks or overheating issues, and the check engine light has stayed off. Before suggesting specific solutions, could you share: Current mileage on your Golf, When the symptoms first appeared, Whether any recent repairs or maintenance were done, If the coolant loss is constant or intermittent, Which specific diagnostic trouble codes are showing This information would help determine if your issue matches what I experienced with the car diagnostics system.

Last service was done 2 months ago and my mileage is 13050km. Thanks for sharing your experience. The control unit diagnostics do show similar errors to what you described. The coolant loss started gradually about 3 weeks ago and becomes worse when the engine is under load. The communication fault appears consistently when scanning, showing error code P0606 (ECM/PCM Processor Fault). The wiring issue you mentioned makes sense, especially since our cars share similar CAN bus architecture. Will have the workshop check the wiring harness connections first before replacing any components. The overheating seems most severe after highway driving, which could point to an electrical rather than mechanical problem. The symptoms match your case quite closely. Will update once the repair is completed.

It sounds like you're dealing with almost exactly what I faced. After seeing your details, I'm even more convinced its a similar wiring issue affecting the CAN bus system. The P0606 code matches what my diagnostics showed, and the behavior under highway load is very familiar. The wiring harness issue in my case was specifically near the firewall connection point, where heat and engine movement had caused wear on the insulation. This created intermittent communication faults between the engine control unit and other modules. One thing I noticed before the repair, the overheating would get especially bad after driving at constant high speeds, just like youre experiencing. This happened because the communication fault prevented proper control of the cooling fans and coolant flow regulation. The workshop should definitely start by checking the wiring harness connectivity around the firewall and engine bay routing. In my car, they initially suspected a faulty control module, but the car diagnostics ultimately revealed it was just the damaged wiring causing the communication fault. Since your mileage is relatively low, this kind of data link issue is likely due to a manufacturing defect or installation problem rather than wear and tear. Keep us updated on what they find.

Thanks everyone for the insights. I had the car checked today and wanted to close the loop on this issue. You were right about the wiring harness, the workshop found damaged insulation near the firewall that was disrupting the CAN bus signals. The constant engine movement had worn through the protective sleeve. The data link connectivity issues were causing the ECM to lose communication with the cooling system control module, explaining both the overheating and the error codes. Total repair cost was 85€ for replacing the wiring section and labor. After fixing the harness and clearing the car diagnostics codes, all symptoms have disappeared. No more coolant leaks, no overheating, and the check engine light remains off. Even after a long highway drive yesterday, everything worked perfectly. Really grateful for the guidance that helped pinpoint the issue. This saved me from unnecessarily replacing control modules when the root cause was just damaged wiring affecting the communication system.