BMW Z4: NOx Catalyst Issues Causing Power & Emissions Woes

Hey there! I have some experience working on cars, and this sounds really similar to what I dealt with on my 2021 BMW M4. I had almost identical symptoms, check engine light, power issues, and that annoying exhaust leak. Took it to my trusted workshop and they found a leak in the exhaust system, specifically around the catalyst area. The whole repair, including parts and labor, cost me about 265 Euro. Quick question though, are you hearing any unusual noises, especially when accelerating? Also, has your fuel consumption increased lately? These details would help me understand if you're facing the exact same issue I had. From what I remember with my M4, the fix wasn't too complex, but it definitely needed professional attention. Let me know those details and I can share more about my repair experience!

Thanks for sharing your experience! Yes, I've noticed a slight whistling noise during acceleration, and my fuel economy has taken a hit recently. I'm at 74194 KM and just had my service last month, so this issue is quite surprising. I'll definitely get it checked by a pro, it's good to know the repair wasn't too complicated on your M4. Did your mechanic need to keep the car for long? I'm hoping to get this sorted quickly since it's my daily driver.

Yeah, I totally get your concern about needing the car back quickly! With my M4, the actual repair work only took about a day once they had all the parts in hand. The whistling noise you're describing is exactly what I experienced, it's like a subtle hiss when you accelerate, right? From my experience, the tricky part wasn't the repair itself but waiting for the parts to arrive. My mechanic ordered a new catalyst and some gaskets, which took about 3 days to get delivered. The whole process, from dropping off the car to picking it up, took around 4 days total. Just a heads up, while they're in there, have them check the surrounding sensors too. When I had mine fixed, they found that one of the oxygen sensors was also starting to fail, which was contributing to the fuel economy issues. Getting everything done at once saved me from another workshop visit later. The car has been running like a dream since the repair, no more whistling, better fuel economy, and definitely more power. Keep us posted on what your mechanic finds. I'm curious to know if it's the same issue I had with my M4!

Thanks so much for all the detailed info! That's really helpful to know about the timing, 4 days isn't too bad, though I'll need to sort out alternative transport. The oxygen sensor tip is particularly valuable, I'll definitely ask them to check that too while they're working on it. The whistling sound is exactly as you described, so I'm feeling more confident now that it's likely the same issue you faced. I've just booked it in with my local BMW specialist for next week. Will have them do a thorough check of everything you mentioned. Thanks again for walking me through your experience, it's really helped ease my mind about the whole situation. I'll be sure to update the thread once I get the diagnosis and repair sorted out!