VW T4 Diesel O2 Sensor Symptoms and Repair Guide

Had a similar issue on my 2004 VW T5 diesel last winter. Engine light came on and fuel consumption went through the roof. As someone with moderate experience working on VWs, I first suspected the O2 sensor based on the symptoms. Took it to my regular workshop where they diagnosed a faulty connection between the oxygen sensor and the wiring harness. The plug had corroded over time, affecting the sensor readings. They cleaned the connections and replaced a damaged section of the wiring for 95€ total, including labor. Job took about an hour. The repair completely resolved both the check engine light and the poor fuel efficiency. The exhaust system started working properly again once the O2 sensor could communicate correctly with the ECU. To better assist with your specific case, could you share: Current mileage on your T4, Any other warning lights or unusual behavior, Whether the issue started suddenly or developed gradually, If you notice any changes in exhaust smell or color This information would help determine if your issue is similar to what I experienced with the wiring or if you might be looking at a different problem with the catalytic converter or sensor itself.

As noted in my original post, my VW has 21848km and just started experiencing these symptoms. Thanks for sharing your experience with the wiring harness issue. Your solution sounds promising and much more cost-effective than replacing the entire oxygen sensor. The air-fuel ratio seems most affected during highway driving, where the poor fuel efficiency is most noticeable. The check engine light came on gradually, flickering at first before staying solid. No unusual exhaust smells or other warning lights present. A workshop quoted 320€ for a complete exhaust sensor replacement, but your post makes me think I should have them check the wiring connections first. The lower repair cost of 95€ for cleaning and wiring repair is certainly appealing compared to full sensor replacement. Have you encountered any recurring issues since your repair was completed?

Thanks for providing those additional details about your T4. Since fixing the wiring issue on my T5, I can confirm there have been zero recurring problems with the oxygen sensor or check engine light over the past year. The gradual onset and highway performance issues you describe match exactly what I experienced. When my sensor wiring was compromised, the ECU would get inconsistent readings during sustained highway speeds, causing the fuel mixture to run rich. Based on your mileage and symptoms, I definitely recommend having the wiring connections inspected before committing to a full sensor replacement. The intermittent check engine light behavior often points to connection issues rather than a completely failed automotive sensor. One thing to watch for, if cleaning the connections doesnt fully resolve the issue, consider having them test the catalytic converter efficiency. Sometimes wiring problems can mask early catalytic converter issues since both components work together in the exhaust system. The 320€ quote seems high if its just the sensor. For comparison, after my successful wiring repair, I had the oxygen sensor tested and it was still within spec, saving me from an unnecessary replacement.

Thanks everyone for the insights on my T4 sensor issue. After reading about the wiring fix, I scheduled an inspection with my workshop focusing first on the connection problems you mentioned. The similarities between our situations, especially the gradual check engine light onset and highway fuel efficiency issues, make me hopeful for a simpler fix. The exhaust system warning signs match up perfectly with what you experienced. My oxygen sensor readings might also be affected by corroded wiring rather than complete sensor failure. The potential 225€ savings between a wiring repair versus full automotive sensor replacement definitely makes it worth checking the connections first. I appreciate the tip about having them verify the catalytic converter while they are inspecting everything. Will update once I know whether cleaning the connections resolved my fuel consumption problems. Its reassuring to hear you had no recurring issues after your repair. One final question, did your workshop include a diagnostic scan with the 95€ repair cost, or was that additional?