VW Lupo Rough Idle & High Fuel Use Points to Knock Sensor

Had a very similar situation with my VW Polo diesel from 2002. When the check engine light came on, I also noticed increased fuel consumption and rough idle. As someone with basic mechanical knowledge, I first tried checking obvious things like loose connections. After monitoring the poor performance for about a week, I took it to my workshop. The diagnostic scan revealed it was indeed a wiring issue, specifically a damaged cable connection to the knock sensor. The constant engine vibration had worn through some of the insulation, causing intermittent signals. The repair was straightforward, they replaced the wiring harness and properly secured it. Total cost was 95€ including the diagnostic work. The ignition timing is now spot-on again, fuel consumption returned to normal, and the warning light cleared up completely. To better assist with your situation, could you share: Current mileage on your Lupo, How long has the warning light been on, Any recent maintenance work done, Does the rough idle happen more when the engine is cold or warm, Any unusual sounds from the engine This information would help determine if your issue matches what I experienced or might be something different.

Last service was done 2000km ago. Since my original post, I took some basic diagnostic steps and monitored the symptoms more closely. The rough idle seems worse when the engine is cold, especially during the first 5 minutes. No unusual sounds from the engine area, but fuel consumption has increased by about 15%. The car problems started exactly 3 weeks ago, coincidentally right after a particularly cold morning. The check engine light remains constantly illuminated, not blinking. Checked for loose connections around the sensor area as suggested, but everything appears properly connected. Based on the repair cost shared, this seems like a reasonable fix if the issue is indeed similar. Will book an appointment to have it properly diagnosed.

Thanks for the additional details. After reading your description, it sounds exactly like what happened with my VW Polo. The cold weather connection is particularly telling, that is when I first noticed my issues too. The reason is that wire insulation tends to contract more in cold temperatures, potentially exposing already damaged areas. The 15% increase in fuel consumption matches my experience almost exactly. The poor engine performance in cold conditions is another typical sign of knock sensor related problems, as the car runs a conservative ignition timing map when it cannot properly detect knock. Since your symptoms started suddenly and match the timing patterns I experienced, I am even more convinced you are dealing with a similar wiring issue. The auto repair should be straightforward once diagnosed. Quick tip before you take it in: make sure to mention the cold weather correlation to the mechanic. This helped narrow down my diagnosis significantly and saved some diagnostic time. The complete fix took about 2 hours in my case, though timing may vary depending on parts availability. Keep in mind that while a wiring issue is likely, only a proper diagnostic scan will confirm the exact cause of your car problems. The good news is that if it is the same issue, it is a relatively simple and affordable repair that will restore your fuel efficiency back to normal.

Based on your description of the cold start issues and consistent malfunction indicator, I am now convinced this is likely similar to your situation with the Polo. I have scheduled a diagnostic appointment for next week and will definitely mention the cold weather correlation. The 15% fuel consumption increase seems to match your experience pretty closely, which is reassuring. Your repair cost of 95€ sounds reasonable, especially considering it resolved all the poor performance issues you described. Thank you for sharing those specific diagnostic details about the knock sensor wiring. It helps knowing that the ignition timing issues can be fixed through a relatively straightforward repair. I will update once I get the official diagnosis, but feeling more confident now about what to expect at the workshop. One final question, did you notice any other warning lights or issues after the repair was completed? Just want to make sure I know what to watch for once it is fixed.