VW Phaeton Water Sensor Issues Causing Engine Problems

As a car owner with some experience in sensor-related issues, I dealt with a similar electrical fault on my 2006 VW Passat last winter. The symptoms you describe match what I encountered, rough running and persistent check engine light, particularly during cold starts. The water sensor fault initially seemed straightforward, but my workshop discovered the real issue was a damaged wiring connection to the sensor, not the sensor itself. The cable insulation had worn through, causing intermittent electrical contact. A proper sensor testing procedure revealed the sensor was actually functioning correctly. The fix involved replacing the wiring harness section and properly securing the connection, total cost was 95€ including labor. Since then, no more sensor fault codes or rough running issues. Before jumping to conclusions about needing a new sensor, would recommend having the wiring thoroughly checked. Could you share: Current mileage? Are there any other warning lights? Does the problem get better once the engine warms up? Have you noticed any coolant loss? This information would help determine if your issue matches what I experienced or if you might be dealing with something different.

Thanks for the feedback. Just to update: my current mileage is 133072km and the last full service was completed 3 months ago. The problem does improve once the engine warms up, but I have not noticed any coolant loss. The moisture sensor readings become more stable after warm-up, suggesting the car diagnostics might be influenced by temperature. The only warning light is the check engine light, no other indicators on the dash. Based on your experience with the wiring harness issue, I will have my mechanic specifically check the connections. The car sensor wiring layout in these Phaetons can be tricky to access, so having a professional trace the electrical fault seems like the sensible next step.

Thanks for the additional details. Your symptoms are matching my experience even more closely now. After reading about the stable moisture sensor readings post warm-up, this really sounds like the same wiring issue I encountered. The fact that there are no other warning lights and no coolant loss is actually good news. When I had my Passat checked, the mechanic explained that moisture sensor issues often trigger false readings during cold starts due to poor electrical conductivity in the damaged wiring. A useful test we did was to gently wiggle the wiring harness near the sensor connection while the engine was running. This caused the malfunction indicator to flicker, confirming the electrical connection problem. You might want to ask your mechanic to perform similar sensor testing. The repair cost should be similar to what I paid (95€), though access might be a bit more challenging in the Phaeton. The important thing is ensuring the new wiring harness gets properly routed and secured to prevent future wear. One thing I learned from my experience: car sensor problems that vary with temperature are often wiring-related rather than sensor failures. When they fixed my wiring, the rough running completely disappeared and the check engine light never came back. Would recommend documenting when exactly the malfunction indicator comes on, this information will help your mechanic trace the fault more efficiently.

Thanks for the detailed responses. The electrical connection theory makes a lot of sense given my symptoms. I had my mechanic check today and they found damaged wiring insulation near the moisture sensor connection, very similar to what was described. They quoted 120€ for the complete repair including new wiring harness section and labor. Will proceed with fixing it next week. Really helpful to know that the malfunction indicator and rough running were resolved by addressing the wiring rather than replacing the sensor itself. The sensor testing they did confirmed what others mentioned, the actual sensor readings were fine once properly connected. The cold start issues and unstable moisture sensor readings were definitely from the poor electrical contact. Will update once the repair is completed, but feeling confident we found the root cause. Big thanks for steering me toward checking the wiring rather than just swapping parts.