VW T5 Engine Woes Point to Faulty Knock Sensor Issue

Having encountered a similar issue with my VW T4 2.0 from 2003, I might be able to help based on my moderate experience with these engines. The symptoms you describe match what I dealt with, check engine light, poor performance and rough idle. My mechanic discovered the knock sensor had a damaged wiring harness connector, causing erratic ignition timing and engine performance issues. The diagnostic showed incorrect readings from the knock sensor, which made the ECU adjust timing incorrectly as a safety measure. The repair involved replacing the wiring connector and testing the knock sensor functionality, totaling 95€ for parts and labor. However, these symptoms could also indicate other issues. To provide more targeted advice, could you share: Have you scanned for error codes? Does the rough idle get worse when the engine is warm? Are there any unusual sounds when accelerating? Has the fuel consumption increase been sudden or gradual? Getting these details would help determine if your issue mirrors what I experienced with my similar engine configuration. Consider having a proper diagnostic test done, in my case, checking the knock sensor signal with an oscilloscope confirmed the wiring issue before any parts were replaced.

Last service was 2 months ago at the same mileage of 44203km, and they didnt spot any issues back then. Your suggested knock sensor diagnosis makes sense considering my car problems. After reading your response, I checked the service records and found that area of the engine was never serviced. The warning light pattern matches what you described. The rough idle definitely gets worse when the engine is warm, and fuel consumption jumped suddenly last week. No unusual sounds during acceleration though. I will request an oscilloscope test at the repair shop to verify if the knock sensor or its wiring is actually causing the malfunction indicator to stay on. Will update once I have the diagnostic results.

Thanks for those extra details, the timeline and symptoms match exactly what I experienced with my VW. When my knock sensor issue occurred, it also came on suddenly despite a recent clean service record. Since you mention no unusual sounds during acceleration but worse idle when warm, this strengthens my suspicion of the knock sensor problem. On my vehicle, the mechanic explained that a failing knock sensor often causes the engine to run excessively rich as a protective measure, leading to that uneven idle and higher fuel usage. The auto repair shop used the oscilloscope test and found the sensor was sending erratic signals, very similar to your situation. The warning light pattern and engine behavior you describe are textbook symptoms of this issue. Quick tip from my repair experience: have them check both the sensor and the entire wiring harness. In my case, they initially only replaced the sensor but the check engine light returned because of damaged wires near the connector. Looking forward to hearing what the diagnostic reveals. The good news is that if it is the knock sensor, it is typically a straightforward fix that resolves all these performance issues quite quickly.

Thanks for sharing those details on your knock sensor repair experience. I can definitely use that information when I take it to the shop tomorrow. What you mentioned about the wiring harness is particularly helpful, I will make sure they inspect both the sensor and the complete wiring. The way you described the rich running condition matches exactly what I am experiencing with my rough idle and increased fuel consumption. Just checked my onboard computer and fuel consumption has increased by about 20% since last week, supporting the theory of the engine running rich. The warning light pattern has been consistently on rather than intermittent, which seems to align with the faulty knock sensor scenario you encountered. I have scheduled an appointment with a repair shop that has the proper diagnostic equipment including an oscilloscope. They will perform the tests first thing tomorrow morning. I feel much more confident now knowing what to ask for and what they should be looking at. Will definitely share the results once I get the diagnosis done.