VW Fuel Problem Points to Faulty O2 Sensor

As someone with some hands-on experience with engine sensors, I had a similar issue with my VW Tiguan (2018, gasoline). The symptoms you describe match what I encountered, poor fuel economy and check engine light. In my case, the O2 sensor was indeed the culprit, but the actual issue was a damaged connection between the oxygen sensor and the engine control unit. The workshop first performed a complete diagnostic scan, which pointed to the exhaust system monitoring circuit. The mechanic noticed the wiring harness leading to the O2 sensor had some wear, causing intermittent connection issues. The repair involved replacing the damaged wiring and connectors, rather than the entire sensor unit. Total cost was 95€ including diagnostic and repair. Before suggesting specific fixes for your case, could you share: Which specific error codes were stored? Have you noticed any unusual exhaust smell? Does the check engine light stay on constantly or flicker? Did fuel consumption increase gradually or suddenly? Your catalytic converter should also be checked, as O2 sensor issues can sometimes indicate broader exhaust system problems. A faulty oxygen sensor can mask catalytic converter issues, so its worth having both components inspected.

Just came from service last week at 141155km where they confirmed the error codes. The check engine light has been constant since then. The exhaust gas smell seems normal, and fuel consumption jumped suddenly about 2 weeks ago. The diagnostic report shows P0171 (System Too Lean Bank 1) and P0135 (O2 Sensor Heater Circuit Bank 1 Sensor 1). I appreciate your detailed response about the wiring, will definitely have them check the connections first before replacing the entire oxygen sensor. Looking at the rise in fuel consumption, something is definitely off with the air-fuel mixture. The exhaust system seems otherwise normal, no unusual noises or visible issues with the catalytic converter. Will schedule another diagnostic check focusing on the wiring harness connections.

Thanks for those additional details about the error codes, they confirm what I suspected based on my experience with my Tiguan. P0171 and P0135 together strongly indicate an O2 sensor related issue, just like what I dealt with. After seeing your codes, I am even more convinced your case mirrors mine. My fuel consumption also spiked suddenly, and the check engine light behavior was identical, constantly on rather than intermittent. Quick word of caution based on my repair experience: while checking the wiring is smart, be prepared that the O2 sensor itself might need replacement. In my follow-up repairs, I learned that the heater circuit failure (P0135) often means the sensor element itself is degrading, even if the wiring looks fine. These symptoms are classic signs of O2 sensor issues affecting fuel efficiency. The engine computer cant properly adjust the air-fuel mixture without accurate sensor input, leading to the lean condition shown by P0171. My eventual fix required both rewiring and sensor replacement, totaling about 280€. The car has run perfectly since, with fuel consumption back to normal. The entire exhaust system health improved noticeably after the repair. Would you mind sharing your current fuel consumption numbers compared to normal? This could help confirm if the severity matches what I experienced.

Following up to share my consumption numbers, since it helps paint a clearer picture. My normal average was around 6.8L/100km, but recently jumped to 8.9L/100km. These numbers make sense given the error codes and lean condition. After gathering all this information about O2 sensors and their impact on air-fuel ratio, I will have the shop check both the wiring and the sensor itself. The 280€ estimate for a complete fix seems reasonable, especially considering the potential fuel savings. The fact that both error codes point to sensor circuit issues, and that they appeared alongside the sudden fuel efficiency drop, makes me confident we are on the right track. Good to know the catalytic converter probably isnt the root cause, though I will have them verify its condition during the repair. Will book the car in next week for the diagnostic work you suggested, focusing on the O2 sensor heater circuit and wiring first. Thanks for sharing your similar experience, its been really helpful in understanding the symptoms and likely repair path.