BMW X3 O2 Sensor Issues: CEL and Failed Emissions Test

Hey there! I have some experience working on BMWs, not an expert but I've dealt with similar issues. I had the same problem with my 2018 BMW X1 last year. The symptoms were exactly the same, check engine light, terrible fuel economy, and emissions problems. My mechanic found that the oxygen sensor's wiring harness had corroded, causing a bad connection. Got it fixed at my local workshop, cost me 95 Euro for the repair, including labor. They had to replace some connectors and clean up the wiring. The whole thing took about 2 hours, and it's been running perfectly since then. Before I can share more specific advice, could you share: Have you scanned for specific error codes? How long has the check engine light been on? Did you notice any other performance changes? Hope this helps! Let me know if you need more details.

Thanks for the input! My X3 has 181205 KM on the clock and this started happening right after my latest service. I got the P0141 code from my OBD scanner, which confirms the O2 sensor heater circuit issue. The check engine light has been on for about a week now, and yeah, I've noticed the engine feels a bit rougher at idle. Fuel consumption jumped from around 8.5L/100km to nearly 11L/100km. Strange that your X1 had a similar issue. Good to know it wasn't too expensive to fix. Did you have any trouble finding parts for yours? I'm hoping it's just a wiring issue like yours and not the entire sensor that needs replacement.

Hey again! Yeah, looking at those symptoms and the P0141 code, it's definitely similar to what I experienced with my X1. When you mentioned it started after the service, that caught my attention, same thing happened to me actually. Wonder if they might've disturbed something while doing other work. In my case, finding parts wasn't really an issue since it turned out to be just the wiring, but I did initially worry about needing a full sensor replacement. When I first took it in, my mechanic was also thinking we'd need to replace the whole sensor unit (which would've cost way more), but thankfully he spotted the corroded wiring first. With your mileage being similar to what mine was (I was at about 175000 KM when it happened), these kinds of issues aren't uncommon. The rough idle is something I experienced too, it's because the engine management system isn't getting the right readings from the sensor to adjust the fuel mixture properly. Quick tip from my experience, if they do need to replace the full sensor, make sure they check both the upstream and downstream sensors while they're at it. Sometimes when one goes, the other isn't far behind. Saved me another trip to the workshop a few months down the line. Have you noticed any weird smells from the exhaust? That was another thing I experienced but forgot to mention in my first reply.

Thanks for all that great information! You're right about checking both sensors, that's really helpful advice. Actually, now that you mention exhaust smells, I have noticed a slightly stronger smell than usual, especially when the engine is warming up in the morning. I've scheduled an appointment with my mechanic for next week. I'm feeling much more confident now knowing what to expect and what to ask them to check. Fingers crossed it's just a wiring issue like yours and not a complete sensor replacement. I'll make sure they inspect both sensors thoroughly while they're at it. Really appreciate you taking the time to share your experience, it's been super helpful to hear from someone who's dealt with the exact same problem. I'll definitely update the thread once I get it sorted out to help others who might run into similar issues with their BMWs.