VW Bora O2 Sensor Issues Impact Fuel Economy

As someone with moderate experience working on cars, I had a similar issue with my 2003 VW Golf, practically the same setup as your Bora. The check engine light came on and I noticed the fuel economy dropping significantly. The problem turned out to be a loose connection at the O2 sensor plug rather than a complete sensor failure. The automotive sensor wiring had worked itself loose over time, causing intermittent connection issues. The exhaust sensor was actually fine, but the faulty connection was making the engine run rich. My mechanic diagnosed it pretty quickly. They cleaned the connector, secured it properly, and checked the oxygen sensor readings to confirm everything was working correctly. Total cost was 95€ for the diagnostic work and repair. You are right to address this promptly. A malfunctioning O2 sensor or connection can definitely cause increased fuel consumption and potentially damage your catalytic converter over time due to incorrect fuel mixture. To help diagnose your specific case, could you share: What error codes are you seeing? Is the check engine light steady or flashing? Have you noticed any rough idling or hesitation during acceleration? Does the problem seem worse when the engine is cold or warm?

Thanks for the detailed response. Just had my latest service done 2 months ago and the issue started right after. The error code showing is P0135, the check engine light stays steady, and the engine runs rough especially when cold. No acceleration issues noticed yet. The air-fuel ratio seems off based on the diagnostics. Given the timing right after the service, could they have damaged the O2 sensor wiring during maintenance? The exhaust sensor readings are inconsistent according to my OBD scanner. Regular car maintenance was always done on schedule, so this sudden issue is concerning. Would appreciate insights on whether I should return to the service center about this.

Having gone through this exact situation with my VW Golf, the P0135 code and symptoms you describe sound very familiar. Since your issue started right after the service, there is a good chance they disturbed the O2 sensor wiring during maintenance. This is quite common when working around the exhaust system. Based on my experience with the same problem, you should definitely return to the service center. The timing of the issue suggests their work may have contributed to the problem. They should check: The O2 sensor wiring harness for any damage or loose connections, The sensor heater circuit (P0135 specifically relates to this), The connection points that were likely accessed during the recent service In my case, the check engine light and rough cold running were resolved once the wiring was properly secured. The exhaust system returned to normal operation and fuel consumption improved significantly within a few tank fills. If they refuse to address it, the repair is fairly straightforward at another shop. The diagnostic and repair cost should be similar to what I paid (95€), especially if its just a wiring issue rather than a failed sensor. Keep monitoring those sensor readings with your OBD scanner. If the values dont stabilize after the wiring is fixed, then you might be looking at actual O2 sensor replacement.

Thanks for the helpful feedback. After reading about your similar experience, I contacted the service center. They acknowledged they had done extensive work around the exhaust system during the last service and agreed to inspect the wiring free of charge. The P0135 code and rough cold starts were exactly what you described. Great to know this might just be a wiring issue rather than a complete O2 sensor failure. I will bring up the specific points you mentioned about checking the heater circuit and connection points during my appointment tomorrow. If they find damaged wiring from the previous service, I expect they should fix it under warranty. If not, the 95€ repair cost you mentioned seems reasonable compared to replacing the entire oxygen sensor. I will keep monitoring the exhaust sensor readings with my OBD scanner after the repair. Knowing that your fuel consumption improved after fixing similar issues gives me hope this will resolve my problems too. Glad I caught this early before it could affect the catalytic converter. Thanks again for sharing your experience, it helped me take the right steps to address this.