VW Passat Hybrid Trans Failure Linked to Cooling System

I had a similar issue with my 2017 VW Golf GTE Hybrid. Having some experience working on cars, I first suspected transmission problems, but it turned out to be related to the cooling system. The symptoms matched yours exactly, transmission warnings, emergency mode, and those worrying grinding sounds. My mechanic found that a faulty pressure sensor in the cooling system was sending incorrect signals, causing the transmission control unit to go into failsafe mode. The diagnostic process involved: 1. Full system scan 2. Cooling fan operation test 3. Pressure testing of the entire cooling system 4. Coolant leak detection check They discovered a damaged wire connection to the coolant pressure sensor, which was causing erratic readings. The repair was straightforward, replacing the wiring harness connector and resealing it properly. Total cost was 95€ including parts and labor. To help diagnose your specific case, could you share: Current mileage, When the symptoms first appeared, Whether the coolant level has changed, If you notice any fluid leaks, Whether the issues occur more during cold or hot weather These details would help determine if your car has the same root cause as mine did.

Thank you for the detailed insight. My mileage is 20914km and had the last service done 3 months ago. The symptoms started appearing gradually over the past week. There is no visible coolant leak under the car, but I noticed the transmission behaves worse when the engine is hot. The cooling fan seems to run more frequently than usual, especially during city driving. What caught my attention in your response was the pressure sensor issue. During the last service they did mention something about checking the cooling system but found nothing concerning at that time. Could the sensor have failed since then? The grinding noise especially worries me as it started after a particularly hot day last week. The transmission warning light is now consistently on, unlike the first few days when it would come and go. Should I get the cooling system pressure tested before looking into transmission repairs?

Based on the running pattern and symptoms you describe, it sounds incredibly similar to what I experienced. The consistent warning light and correlation with engine temperature strongly point toward the cooling system pressure sensor, just like in my case. The fact that your transmission behaves worse when hot and the cooling fan runs excessively suggests the pressure sensor might be sending faulty signals to the transmission control unit. These sensors can fail suddenly, even shortly after a service check. When my car exhibited these symptoms, the mechanic conducted a pressure test that revealed inconsistent readings from the sensor. The grinding noise in your case matches what I heard, it happened because the transmission was receiving incorrect pressure data and adjusting inappropriately. Since we have nearly identical symptoms, I strongly recommend getting the cooling system checked first. The pressure gauge test costs far less than transmission repairs, and in my case, it revealed the true problem quickly. A faulty pressure sensor can trigger multiple error codes that appear transmission-related. Request a complete cooling system diagnostic, including: Pressure sensor electrical testing, Radiator repair inspection, Complete coolant leak detection, Cooling fan cycle testing This systematic approach saved me from unnecessary transmission work, and the sensor replacement resolved all symptoms.

Thanks for the informative reply. I will definitely prioritize getting the cooling system checked first, especially since my symptoms match your experience so closely. I just made an appointment for tomorrow morning and specifically requested a full cooling system diagnostic with pressure testing. The fact that the transmission issues worsen with heat and the cooling fan runs excessively does suggest a connection to the cooling system, rather than a pure transmission problem. I will ask them to check the radiator coolant pressure sensor thoroughly, since this was the culprit in your case. The grinding noise has me worried, but knowing that yours was caused by the same issue gives me hope that this might be a simpler fix than a full transmission repair. Will update once I get the results from the diagnostic tests. I am hoping it turns out to be just a faulty sensor or wiring issue like in your case. The car overheating and transmission warnings are quite stressful, but at least I now have a clearer direction for the repairs.