Bad O2 Sensor Warning Signs: Fuel Waste and Check Light

I had a similar issue with my 1998 GMC Jimmy. With some basic experience working on cars, I initially thought it was the catalytic converter causing problems. After taking it to a workshop, they discovered it was actually just a damaged wire connection to the O2 sensor. The exhaust sensor itself was functioning properly, but the connection had deteriorated over time. The repair cost 95 Euro including labor, which was a relief since I was worried about needing to replace the entire exhaust system. The symptoms were exactly like yours, increased fuel consumption and check engine light. After fixing the connection, fuel economy went back to normal and the warning light disappeared. To better assist with your specific case, could you share: Which error codes your diagnostic showed, Whether you notice any unusual exhaust sounds, If the engine runs rough at idle, Your current MPG compared to normal These details would help determine if your issue matches what I experienced or if it might be something else in the oxygen sensor system. The workshop mentioned that damaged sensor connections are fairly common on these models, but its always best to get a proper diagnosis since similar symptoms can have different causes.

Thanks for your feedback. My most recent service was at 114022 KM where they also checked the air-fuel ratio sensors. The diagnostic showed P0135 and P0141 codes. The engine runs smoothly at idle, but I do notice some hesitation during acceleration. There are no unusual exhaust sounds. My fuel consumption has increased from about 14 to 18 liters per 100 km in the past month. The wire connection issue you mentioned is interesting, my exhaust system looks pretty worn overall, so damaged wiring could definitely be possible. Will have a professional check those connections before replacing any O2 sensors. Good to know it might be a simpler fix than expected.

Thanks for providing those details. Given the specific error codes P0135 and P0141, your case actually seems different from what I encountered. These codes point to heating circuit issues in both oxygen sensors, unlike my simple wire connection problem. The smooth idle but hesitation during acceleration, combined with the significant jump in fuel consumption, suggests the sensors themselves might be failing to properly regulate the air-fuel ratio. In my GMC Jimmy, the hesitation wasnt present, it was more of a constant fuel efficiency issue. While checking connections is still worthwhile, the error codes strongly indicate the actual sensors might need replacement. My repair was simple because only the wiring was affected, but based on your codes and symptoms, you might be looking at a more extensive fix. A professional inspection is definitely the right move here. The combination of error codes, increased fuel consumption, and acceleration issues matches more closely with failing oxygen sensors rather than just connection problems. When they check it, have them inspect the entire exhaust system since you mentioned its overall worn condition. Would be interested to hear what they find, especially since these symptoms are common across our GMC models but can have different root causes affecting the air-fuel ratio system.

The symptoms you both described sound very familiar to what I experienced. After those diagnostic codes and details about fuel consumption, I decided to have everything checked at a repair shop yesterday. The technician confirmed both O2 sensors needed replacement, noting the heating circuits had failed. The total repair cost was 320 Euro for both sensors plus labor. They also discovered some minor exhaust system corrosion but said it wasnt affecting performance yet. After the sensor replacement, the check engine light cleared and the acceleration is much smoother now. Fuel consumption has already dropped back to normal levels. Looks like the automotive sensor diagnosis was spot-on, definitely worth getting it properly checked instead of just assuming it was wiring like in your case. Thanks again for sharing your experience. It helped me understand the issue better and gave me confidence to get it checked properly rather than trying cheaper fixes first that might not have solved the actual problem with my catalytic converter system.