BMW M3 O2 Sensor Issues: High Fuel Usage Alert

I had a similar issue with my 2016 BMW M4 last summer. The symptoms were exactly the same, check engine light and increased fuel consumption. Initially thought it was the O2 sensor too, but when I got it checked, turned out the connection cable to the sensor was damaged and had some corrosion. The connector wasn't making proper contact, causing wrong readings. The mechanic showed me how the cable insulation had worn out near the connection point, and there was visible damage. It wasn't a major repair, just needed to replace the wiring harness and clean the connector. The car's been running perfectly since then, back to normal fuel consumption. Honestly, this was more of an annoyance than a serious issue, but it's important to fix it quickly because bad readings can affect your engine's performance and fuel efficiency. If you let it go too long, it could potentially cause other problems. Worth getting it checked soon, these things don't fix themselves.

Thanks for sharing your experience! That's really helpful to know it wasn't the O2 sensor itself. Just curious, do you remember roughly how much you ended up paying for that wiring harness fix? Also, has everything stayed working fine since then, or have you noticed any other quirks?

Hey again! Happy to give you an update. I just got my M4 back from the mechanic last week, turned out it was exactly what I had suspected from my previous experience. The aged oxygen sensor was causing the trouble, but thankfully the fix was straightforward. The total bill came to 95 € for fixing the cable and plug connection that was acting up. Everything's been running like a dream since the repair. My fuel consumption is back to normal, and that annoying check engine light is finally gone. I've put about 500 km on it since the fix, and there haven't been any quirks or issues at all. Just a heads up, even though my previous problem was with the wiring harness, these symptoms can point to different issues in different cases. Good thing you're getting it checked out now before it potentially causes any bigger problems. Let us know how it goes with yours, always interested to hear if it turns out to be something similar!

Thanks everyone! Just got back from the mechanic, and you all won't believe it, turned out to be a faulty fuel injector that was sticking open! No wonder my fuel consumption was through the roof. The mechanic showed me the old part, and it was pretty gunked up. Had to fork out quite a bit more than expected, around 400 bucks for parts and labor. But honestly, seeing how much I was wasting on extra fuel, it'll probably pay for itself soon enough. They also cleaned out the other injectors while they were at it, which was a nice touch. Car's running super smooth now, and I can already tell the difference in fuel consumption. Filled up yesterday and the gauge is barely moving compared to before. Really glad I didn't just ignore the problem. Thanks again for all your input, it really helped me push through the anxiety of dealing with another mechanic. This new place seems pretty solid, might stick with them for future maintenance too.