VW Golf Diesel O2 Sensor Causing Poor MPG & CEL

Had this exact issue with a 2004 VW Jetta TDI. The malfunction indicator light came on and fuel consumption increased noticeably. The diagnostic scan revealed a faulty oxygen sensor heater element. The problem was more complex than just a bad sensor. The wiring harness connecting to the oxygen sensor had corroded, causing intermittent connection issues. When the heater element fails or gets interrupted signals, the engine computer defaults to a rich fuel mixture as a protective measure. The repair required: Complete inspection of the exhaust system, Testing the car components around the O2 sensor, Replacing the corroded wiring harness, Installing a new oxygen sensor, Clearing the check engine light codes, Performing a sensor calibration This was a serious issue that needed immediate attention since it affected both emissions control and fuel efficiency. The symptoms matched perfectly, poor fuel economy and persistent check engine light. Would strongly recommend having a full diagnostic done rather than just replacing parts, as the underlying wiring problem could damage a new sensor if not addressed. These VW diesel sensor circuits are known to develop issues around the 130-150k km mark. Getting proper diagnosis and repair from a shop familiar with these systems is essential.

Thanks for sharing that detailed experience. Just encountered something very similar with my Golf recently. Really helpful to know about the wiring harness issue, as I might have just replaced the oxygen sensor without checking that. Was wondering how much the complete repair ended up costing you? And has everything been running smoothly since the fix, or have you noticed any other sensor-related problems? Just want to get an idea of what to expect cost-wise and if I should be looking out for other potential issues while the car is in the shop. The malfunction indicator light has been bugging me for weeks now, and the fuel consumption is definitely worse than usual. Planning to take it in next week, so any additional insights would be great.

After getting it properly diagnosed and fixed, the total cost came to 95€ for repairing the cable connection to the oxygen sensor. The air-fuel ratio returned to normal immediately after the repair. Turned out the issue was exactly what the sensor testing showed, just a faulty connection causing the oxygen sensor heater failure. The malfunction indicator light cleared right away and has stayed off since the repair. Fuel economy went back to normal within the first tank after fixing it. The mechanic did a thorough check of the surrounding components while fixing the connection, and everything else looked good. The biggest takeaway was the importance of proper diagnosis. A complete system check revealed it was just the connection issue rather than needing a full sensor replacement, which would have cost significantly more. The check engine light hasnt returned in the months since the repair, and the cars been running perfectly. If youre heading to the shop next week, make sure they inspect the wiring harness and connections before jumping to replace the entire sensor. Often these issues can be fixed for much less than a full sensor replacement.

Thanks everyone, wanted to share how my repair experience turned out. Took the car to a specialized VW shop after reading the responses here. Really glad I did since they found the exact same wiring harness corrosion issue mentioned. The full diagnostic test confirmed the faulty sensor reading was due to damaged wiring, not a bad sensor. Total repair came to 120€ including new wiring harness and labor. The check engine light turned off right after they fixed it. Best part is the fuel economy is back to normal now. The technician showed me the corroded wiring and explained how it was causing the malfunction indicator to stay on. They also checked surrounding components while doing the repair, all looked good. Much cheaper than the 400€+ I was quoted elsewhere for a complete oxygen sensor replacement. Vehicle emissions are testing normal again and everything is running smoothly. Really appreciate the advice about getting the wiring checked first instead of just swapping parts.