Z3 O2 Sensor Failure Draining Gas & Failing Emissions

Hey there! I can relate to your O2 sensor troubles. I had similar issues with my 2003 BMW M3 last year. The symptoms were exactly what you're describing, high fuel consumption, check engine light, and failed emissions test. Got it checked and turned out the connector plug on the downstream O2 sensor was corroded and damaged. The wiring harness also had some wear. It wasn't a super serious repair, but definitely not something you want to ignore since it affects both fuel economy and emissions. The mechanic showed me that the heater circuit was dead, which was causing incorrect readings. They replaced the sensor and fixed the wiring connection. Car's been running great since then, fuel consumption back to normal and no more check engine light. Given your symptoms, I'd definitely get those O2 sensors checked out, especially the wiring and connections. It's worth finding a mechanic who knows BMWs well for this kind of work.

Thanks for sharing your experience! That's really helpful to know it was the downstream sensor in your case. I've been worried about repair costs since these German cars can get expensive to fix. Do you remember roughly how much you ended up paying for the sensor replacement and wiring work? Also, has everything stayed working properly since the fix, or have you had any other sensor issues pop up?

Hey again! Yeah, absolutely happy to update you! I actually just got mine fixed last week, and it turned out to be exactly what I suspected, the oxygen sensor heater was the culprit. The total bill came to 95 € for fixing the cable and plug connection. Honestly, I was relieved it wasn't something more serious! The good news is my M3 is running like a dream now. The check engine light is gone, and fuel consumption is back to normal. It's amazing what a properly functioning O2 sensor can do! Haven't had any issues since the repair, and my mechanic did a thorough job explaining what was wrong. You'll probably be happy to hear it wasn't as expensive as you might fear. Like you said, German cars can hit the wallet hard sometimes, but this was actually pretty reasonable. Just make sure you find someone who really knows BMWs, it makes all the difference. My guy spotted the issue right away because he's seen it before on similar models. Feel free to ask if you need any other details about the repair. Always happy to help out a fellow BMW owner!

Thanks again for all the info! I bit the bullet and got my Z3 checked out yesterday. You were right about finding a BMW specialist, what a difference! The mechanic diagnosed it right away, turned out my front O2 sensor was completely shot, and there was a small wiring issue too. Bill came to around 120 € total, which wasn't too bad considering all the horror stories I'd heard. Car's already running so much better, the engine feels smoother, and I can tell it's not guzzling fuel like before. The check engine light is finally off, which is such a relief. Really glad I didn't put this off any longer. Your experience definitely gave me the push I needed to get it sorted. I'm keeping this mechanic's number handy for future reference, seems like a trustworthy guy who knows his stuff. Thanks for helping me avoid another frustrating experience with my previous mechanic!