M3's Start-Stop Nightmare: Engine Dies While Driving

I had a similar issue with my 2009 BMW M5 last month. The battery was severely degraded and couldn't hold enough charge to support the start-stop system. It also caused unusual engine shutoffs while driving, which was quite dangerous. The diagnostic showed multiple electrical system errors. When I took it to the garage, they found the battery had failed several cells and was operating at about 30% capacity. They had to replace the battery and recalibrate the car's electrical system. This wasn't just a simple battery swap, the new one needed proper coding to work with the car's computer system. The job was serious because failing batteries in these cars can damage other electrical components if left untreated. My mechanic also cleaned the battery terminals and checked the alternator to ensure it was charging correctly. From your description, it sounds very similar to what I experienced. I'd suggest getting the battery tested first, it's usually the root cause of these symptoms in BMWs.

Hey, yeah that's spot on with what I experienced! My battery was also the culprit, sounds exactly like my issue. The garage did a full electrical system reset after installing the new battery too. I'm curious, do you remember how much you ended up paying for the whole fix? Have you had any other electrical gremlins pop up since they did the work? My car's been running smooth for a while now, but you know how these BMW electronics can be sometimes.

Hey again! Glad to hear you got it sorted too! After seeing how you dealt with the same thing, I can confirm the repair totally fixed my start-stop issues. The whole thing, including the battery replacement and system recoding, came to 290 €. Since the fix, my M5's been running like a charm, no more unusual shutdowns or warning lights. The start-stop system works perfectly now, which is such a relief. It's funny how such a simple thing like a degraded battery can cause so many weird problems in these cars! I've had zero electrical issues since the repair. You're right though, these BMW electronics can be tricky, but when everything's working properly, they're amazing machines. The key was getting that battery sorted before it could mess up other components. Trust me, it was worth every cent to get it properly fixed. These cars really need proper coding when replacing batteries, learned that the hard way! How's your M3 holding up now? Sounds like we both dodged a bullet by getting it sorted quickly.

Thanks for sharing your experience! It's such a relief to hear your repair cost was similar to mine. My fix came in at around 300€, so pretty much the same ballpark. I actually just got back from the garage last week after they did the work. The mechanic I found was really thorough, explained everything clearly, and even showed me the old battery's condition. Pretty scary to see how degraded it was! They did the full coding and reset, and honestly, the car feels better than it has in months. No more heart-stopping moments with unusual shutdowns, and that annoying warning light is finally gone. The start-stop system works perfectly now, it's amazing how one failing component can cause so many issues. The engine runs smoother too, which I wasn't expecting. I'm just glad I asked around and found someone who knew what they were doing. My old mechanic would've probably just swapped the battery without the proper coding and called it a day. Sometimes paying a bit more for someone who really knows these cars is worth it!