BMW Z3 Check Engine + High Fuel Use: O2 Sensor Issue?

Hey there! I had a similar issue with my 1998 BMW Z4 last year. I have some experience working on cars, mostly basic maintenance stuff. When my check engine light came on and fuel economy went downhill, I took it to my regular workshop. Turned out it was just a loose connection at the oxygen sensor plug, the cable had worked itself free over time. Fixed it for 95 Euro, which included diagnostic and labor. The mechanic cleaned the connector and secured it properly. After that, the check engine light went off and fuel consumption went back to normal. Could you share what error code you're getting? Also, have you noticed any rough idling or hesitation during acceleration? This info would help determine if you're dealing with the same issue I had. By the way, if it helps to know, my old E46 had a similar problem once. These sensors can be tricky on BMWs.

Thanks for sharing! I'm fairly new to BMW ownership, just 7727 KM on my Z3 since the latest service. My symptoms sound exactly like what you described. I'm also getting rough idling, especially when the engine is cold. The error code is P0135, and yes, there's definitely some hesitation during acceleration. I was worried it might be something major, but hearing about your experience gives me hope it could be just a connection issue. My only concern is that my local mechanic isn't too familiar with BMWs. Would you mind sharing if your mechanic did anything specific to diagnose the loose connection before jumping straight to sensor replacement?

Hey again! Absolutely happy to share more about my experience! You've described the exact same symptoms I had, that rough cold idle really takes me back. When I see P0135, I immediately think O2 sensor circuit, since that's what gave me trouble. My mechanic actually showed me his diagnostic process. First, he checked the wiring, that's how we caught the loose connection in my case. He basically wiggled the connections while monitoring the sensor readings. The readings would jump all over the place when he moved the connector, which was the dead giveaway. Given that you're at 7727 KM since your last service, I'd definitely suggest getting those connections checked first before replacing anything. My BMW was super sensitive to any loose electrical connections, especially around the O2 sensors. Quick tip from my experience, if your mechanic isn't BMW-savvy, maybe mention checking the connector's pins for corrosion too. That was actually part of my problem that I forgot to mention earlier. The pins needed a good cleaning before reconnecting everything. Just make sure they don't jump straight to replacing the sensor without checking these basics first. Could save you some serious cash, just like it did for me! Let me know how it goes, I'm really curious if you've got the same issue I had!

Thanks so much for the detailed explanation! This is incredibly helpful. I've just called my mechanic and specifically asked about checking the connections and pins before replacing anything. He seemed receptive to the suggestion and actually mentioned he has a diagnostic tool that can monitor the sensor readings in real-time. I've booked the car in for tomorrow morning, and I'll make sure they follow the diagnostic steps you described. It's such a relief to know this might be a simple fix rather than a complete sensor replacement. I'll definitely keep an eye on those connections during future services too, seems like it's a common weak point in these cars. Thanks again for taking the time to share your experience. I'll update the group once I get it sorted out. Fingers crossed it's just a loose connection like yours was!