VW Passat Hybrid Cooling System Failure and ECU Issues

As someone with moderate experience working on hybrid vehicles, I had a very similar issue with a 2017 VW Golf GTE. The symptoms you describe match almost exactly what I encountered, including the overheating and coolant leaks despite proper fluid levels. After running car diagnostics, my workshop found a faulty wiring connection between the hybrid control unit and the engine management system. The diagnostic trouble code pointed to a communication error in the CAN bus network, which was causing the cooling system to operate erratically. The fix involved replacing a damaged connector and repairing the wiring harness where it had worn through near the transmission. Total cost was 95€ for parts and labor. Since the repair, the system has worked flawlessly with no more overheating or leaks. Before suggesting specific fixes though, could you share: Have you noticed any pattern when the overheating occurs? Are there any other warning lights besides the check engine light? Does the issue happen more in electric or hybrid mode? Have you had any recent work done on the cooling system? This information would help determine if your issue matches what I experienced with the control unit communication problem.

Correction, mine is also at 142459km and was just serviced last month. The software glitch diagnosis matches my situation exactly. After the latest control module scan, they also found additional communication fault codes I hadnt seen before. The workshop initially replaced the coolant temperature sensor but that didnt resolve it. The CAN bus error kept showing up in repeated car diagnostics. Going back next week to have them check the wiring harness connections as you suggested. Would appreciate if you could share more details on which specific connectors were faulty in your case. Did the repair require any control module reprogramming after fixing the wiring?

Thanks for the follow-up. Definitely seeing more parallels between our cases now. At that same mileage, mine showed nearly identical diagnostic trouble codes pointing to the CAN bus communication fault. In my case, the main culprit was the connector block linking the hybrid control unit to the engine management module. The wiring issue specifically involved pins 12 and 14 in the main harness connector, which had developed resistance due to corrosion. The workshop found this after doing a thorough pin-to-pin continuity test. The repair did require control unit reprogramming after fixing the physical connection. The software glitch between the hybrid and engine systems persisted until they performed a complete module reset and recalibration. This extra step was crucial, just fixing the wiring alone didnt fully resolve the communication fault. Total resolution involved: Replacing the main connector block, Repairing chafed wiring near the transmission tunnel, Full control module reprogramming, System adaptation procedure Post-repair car diagnostics showed all communication restored between modules. The cooling system pressure normalized and no more coolant leaks occurred. Keep us updated on what your workshop finds in the wiring harness inspection.

Thanks for the detailed info. Those connector locations match exactly what my mechanic was suspecting might be the connectivity issue. After seeing so many matching symptoms between our cases, I feel more confident now about the repair direction. I scheduled the full diagnostic check for next week focusing specifically on those connector pins and the transmission tunnel wiring you mentioned. The connectivity issue with those particular pins seems to explain the irregular cooling system behavior perfectly. Given how similar our problems are with the diagnostic trouble codes and software glitch patterns, I will make sure they include both the wiring repair and complete control module reprogramming in the repair plan. Hopefully this resolves the persistent communication fault that keeps triggering the overheating. Will update once they complete the car diagnostics and wiring inspection. Really helpful to know about the specific fix that worked in your case, gives me a much clearer picture of what to expect from the repair process.