VW Up Rough Idle Points to Faulty Knock Sensor Issue

I experienced similar symptoms with my 2014 VW Polo with the 1.2 TSI engine. The check engine light and poor performance issues were almost identical to what you describe. After some initial troubleshooting, my workshop found that the knock sensor wiring harness had a damaged connection. The sensor itself was fine, but the faulty wiring was causing incorrect readings. The total repair cost was 95€, which covered diagnostics and fixing the cable connection. For my case, the workshop spotted the issue fairly quickly using their diagnostic equipment. The mechanic mentioned these symptoms can also be caused by: Faulty fuel injectors, Spark plug issues, Vacuum leaks, Mass airflow sensor problems Before suggesting more specific solutions, could you share: Your engine type and size, Current mileage, Whether you notice the rough idle more when the engine is cold or warm, If you have access to a diagnostic scanner to read the error codes This information would help determine if your issue matches what I encountered or if it might be something different.

Last service was done at 128000km and currently showing 130859km. The mileage seems relevant since it lines up with when these auto repair issues started. My ignition timing also feels off, which I forgot to mention in my initial post. The engine misfires mainly when cold, plus the fuel efficiency has dropped from 5.2L/100km to around 6.8L/100km on my usual routes. Really hoping it is just the knock sensor and not something more serious affecting engine performance. The car problems are most noticeable during the first 5-10 minutes of driving. After warmup the rough idle improves somewhat but the check engine light remains on. Unfortunately I do not have access to a diagnostic scanner at home.

Thanks for providing those details. Based on your description, especially the cold-start issues and timing problems, I am less convinced it is just the knock sensor like in my case. The combination of ignition timing issues, increased fuel consumption, and cold-start misfires points more toward a potential ignition coil or spark plug problem. In my VW, I initially thought it was the knock sensor too, but after getting proper diagnostics, we found multiple issues: Worn spark plugs causing misfires, Carbon buildup affecting idle quality, Slightly stretched timing chain The total repair ended up around 450€, including parts and labor. While more expensive than just a knock sensor repair, fixing these underlying issues completely resolved the rough running and high fuel consumption. Given your mileage and symptoms, I would strongly recommend getting the ignition system checked first. The fact that the malfunction indicator improves after warmup is a classic sign of ignition-related car problems rather than a knock sensor issue. Would suggest having a workshop do a proper diagnostic scan to confirm. They can measure actual ignition timing and check for misfire codes, which will pinpoint the exact problem areas.

Thanks for the detailed response. After hearing about your similar experience with ignition timing issues, I will definitely get my car properly checked rather than just assuming it is the knock sensor. Yesterday I took a longer highway trip and noticed the fuel efficiency was even worse than around town, nearly 7.5L/100km. This seems to support your theory about ignition system problems potentially causing the increased consumption. I will book the car in for a proper diagnostic scan next week. The 450€ total repair cost you mentioned seems reasonable if it resolves both the engine knock and cold start problems completely. Much better to fix the root cause than keep driving with poor performance and risk more damage. Will post an update once I know what the actual issue is. Thanks again for helping clarify that these symptoms likely point to ignition system troubles rather than just a faulty knock sensor.