VW Fox Diesel: Water Sensor Causing Rough Running

Having some experience with VW diesels, I faced almost identical symptoms on my 2008 VW Polo TDI. The malfunction indicator light came on and the engine ran terribly, especially during cold starts. The electrical fault turned out to be a damaged wire connection at the water sensor. The sensor itself was working fine, but moisture had corroded the plug connection over time. The sensor testing confirmed the component was good, but the electrical signals weren't reaching the control unit properly. The repair was straightforward, the workshop cleaned the connection points and replaced the damaged part of the wiring harness. Total cost was 95€ including diagnostic time. The rough running cleared up immediately after the fix. To better assist with your case, could you share: Current mileage on your Fox, How long has the malfunction indicator been on, Any other warning lights showing, Have you noticed any changes in fuel consumption, Did the problem start suddenly or develop gradually These details would help determine if your issue matches what I experienced with the sensor fault on my Polo.

Had my car in for service last month at 142031km. After discussing this with my mechanic, turned out my issue was a bit different. The car diagnostics revealed a faulty connection at the water sensor, but the sensor itself was actually working properly. My mechanic ran moisture sensor tests and found the signals were getting interrupted by corrosion at the plug. Once they fixed the wiring and cleaned the connection, the rough running disappeared completely. They had to replace part of the harness, but the auto electronics repair was much simpler than expected. The total came to 120€ including parts and labor. The car runs perfectly now and the malfunction light hasn't come back on. The cold start issues are completely gone too.

Thanks for the follow-up. Your repair experience matches what I found with my Polo sensor fault exactly. The electrical connection issues on these VW water sensors are quite common, especially at higher mileages. The car diagnostics on my Polo initially pointed to a complete sensor replacement, but like in your case, a thorough check revealed just corroded wiring. Your repair cost was close to mine at 120€ vs 95€, which seems reasonable considering parts prices have increased. Good that your mechanic did proper testing before replacing parts. Too often workshops just swap the entire sensor when the malfunction indicator appears, which can be unnecessary if the issue is just in the wiring. Since fixing the connection in my Polo, I have not had any recurring electrical fault codes or rough running. If you want to prevent future issues, consider having the wire connections checked during regular service intervals, especially around moisture-prone areas where sensor fault problems typically start.

Thanks for the advice. Just wanted to wrap this up with my final experience. The auto electronics repair ended up being straightforward, exactly like others described. The malfunction indicator came on due to corroded wiring at the moisture sensor connection, not a complete sensor failure. The repair cost was reasonable at 120€, covering both parts and labor. My mechanic did a proper job testing the electrical fault before replacing anything. They cleaned the connections and replaced a section of the wiring harness. The water sensor itself was actually fine. Now the engine runs perfectly, cold starts are smooth, and the malfunction indicator hasn't returned. For anyone searching with similar symptoms, definitely get the wiring connections checked first before replacing the entire sensor unit. The fix might be simpler and cheaper than expected. This forum helped me save money by knowing what to ask the mechanic to check. Glad I didn't immediately assume I needed a complete sensor replacement when the real issue was just corroded wiring.