Sprinter engine knock and fuel inefficiency

Based on your description, the symptoms do point towards a knock sensor issue, specifically the uneven idling and the check engine illumination. The diagnostic trouble code (DTC) should confirm this. A similar scenario occurred with my previous BMW 3 series. In my case, the increased fuel consumption was quite noticeable. I would suggest you inspect the knock sensor wiring harness for any signs of damage, corrosion or loose connections. Use a multimeter to check the continuity of the wiring from the sensor to the engine control unit (ECU). Have you had the error codes read?

Thanks for the advice! I haven't checked the continuity yet, but I did visually inspect the wiring and connectors. Everything looks okay at first glance, but I will check the continuity with a multimeter. Regarding the error codes, yes, I read them. It's throwing a code specifically for the knock sensor. Could a faulty ECU trigger the same symptoms even if the sensor itself is fine?

While a faulty ECU could theoretically cause similar symptoms, it's far less likely than a problem with the knock sensor or its associated wiring. Given that you're getting a specific knock sensor error code, I'd advise you to focus your troubleshooting efforts there. If the wiring checks out, and the sensor is indeed defective, replacement is the next logical step. Given the age of the vehicle, I would recommend a visit to the workshop.

Thanks so much for the help! I followed your advice and after checking the wiring thoroughly, I took it to a local shop. Turns out it was indeed the knock sensor, just as suspected. The cable connection was defective. They replaced it, and the check engine light is now off, the engine knock is gone, and the idling is smooth again. The whole thing cost me around 95€. Fuel efficiency is back to normal too!