Audi S4 Knock Sensor Issues Impact Performance & MPG

As someone with some experience working on Audis, I had a similar issue with my 2002 Audi A4 1.8T. The symptoms you describe match exactly what I experienced, poor performance, increased fuel efficiency issues, and that persistent warning light on the dashboard. After running diagnostics, my mechanic found that the knock sensor connection was loose, with corroded wiring at the plug. The check engine light was triggered by this faulty connection. The fix involved cleaning the connector and replacing some damaged wiring, total cost was 95 Euro and took about 2 hours to complete. However, before jumping to conclusions, it would help to know: Have you had the diagnostic codes read? What engine size does your S4 have? Did these symptoms appear gradually or suddenly? Does the engine performance improve after warming up? The knock sensor is crucial for engine protection and optimal performance. A faulty one can definitely cause the symptoms you describe, but similar issues could also stem from other components. Getting the exact diagnostic codes would help narrow down the problem. If you want more specific advice about your S4, these details would help determine if your issue matches what I experienced with my A4.

Thanks for the helpful information. Just had my regular service done at 176397KM so all filters and plugs are new. The engine knock started gradually over the past month. When warm, performance stays poor and the engine keeps running rough. I checked the codes yesterday and got P0325, which points directly to the knock sensor circuit. The symptoms match perfectly with what you described, poor fuel efficiency and that annoying check engine light. Will have a mechanic check the wiring connection first, as that seems like a smart starting point before replacing parts. The corrosion issue you mentioned makes a lot of sense given how these sensors are positioned on these engines.

Thanks for the additional details. Since you mentioned getting the P0325 code, it definitely reinforces my initial suspicion about the knock sensor being the culprit. Having dealt with this on my A4, the gradual onset of symptoms is very typical of a failing knock sensor setup. The fact that your symptoms persist when warm rules out several other potential causes. With my car, the poor performance and rough idle remained constant regardless of engine temperature, just like yours. One thing I noticed after fixing the knock sensor connection on my A4: the fuel efficiency improved dramatically within the first tank of gas. The malfunction indicator light cleared up immediately, and the engine smoothed out completely. Since your regular service is up to date, focusing on the knock sensor circuit is the logical next step. The labor costs should be reasonable since accessing the sensor mainly involves removing the intake manifold cover. If the wiring turns out to be the issue like in my case, you might save quite a bit compared to a full sensor replacement. Let us know what the mechanic finds. Based on the identical symptoms and that specific trouble code, I bet youll see similar positive results once the knock sensor issue is resolved.

Thanks for the advice. Just got back from the mechanic and wanted to share the outcome. The knock sensor wiring had indeed corroded, causing all those issues with poor performance and the constant malfunction indicator light. They cleaned and repaired the connection without needing to replace the whole sensor. Total repair cost was 120 Euro for labor, which seems fair. The engine knock is completely gone now and fuel efficiency has already improved noticeably on my drive home. The check engine light cleared after they reset the system, and the rough idle has disappeared too. Really glad I checked here first before assuming I needed a complete sensor replacement. This saved me both time and money. The gradual decline in performance definitely matched what others experienced with similar knock sensor issues. For anyone else facing these symptoms, definitely worth checking the wiring connections first.