BMW 1 Series Won't Start: Dead Battery or ECU Issue?

Hey there! I had a similar issue with my 2019 BMW 3 Series diesel last winter. Started with dim lights and rough starts, turned out the original problem was a faulty alternator that wasn't properly charging the battery. The dead battery then caused the car's computer to store error codes. The tricky part was that the first mechanic only replaced the battery without checking the alternator. Two weeks later, same symptoms returned. Had to get it properly diagnosed at another shop where they found the alternator issue. They also had to reset the electronic control units since they got scrambled from the power issues. This was actually quite serious because running with a bad alternator could've left me stranded anywhere, plus the constant power fluctuations aren't good for the car's electronics. Would definitely suggest getting the charging system fully tested, both battery and alternator, rather than just assuming it's a battery issue. These electrical problems tend to cascade if not fixed properly the first time. Good luck with yours.

Thanks for sharing your experience! This is super helpful as it sounds exactly like what I'm dealing with. Quick question, do you remember roughly how much you ended up paying for the full fix? And has everything been running smoothly since then, or have you noticed any other electrical quirks pop up? Just want to get an idea of what I might be in for cost-wise and if I should watch out for any lingering issues.

Hey again! Happy to give you an update! Since getting it properly fixed, my 3 Series has been running perfectly, no more starting issues or electrical gremlins whatsoever. The mechanic discovered it was indeed a consequential error from the previous work, so they actually fixed it free of charge. Turns out the electronic control unit just needed a proper reset after the alternator replacement. It's been about a year now, and I haven't had a single electrical quirk since. The car starts first time, every time, even in cold weather. The lights are bright and steady, and all the electronics work exactly as they should. Honestly, it's like having a new car again. My best advice would be to insist on a full diagnostic of the charging system. Make sure they check both the alternator and battery, plus all the connections. Sometimes what seems like a big electrical problem can be sorted quite easily if you find the right mechanic who knows what they're looking for. Let me know how you get on with yours!

Thanks everyone for the help! Wanted to give you all an update on what happened. Finally got my 1 Series sorted out. Took it to a new mechanic who specializes in BMW electronics. They found the alternator was indeed failing, just like some of you suggested. Also had a corroded connection that was making things worse. The whole repair ended up taking about a day. Got charged for the alternator, labor, and diagnostic testing. The mechanic was really thorough and even showed me the worn-out parts. Car's running like new now, starts instantly, lights are bright, and all those error codes are gone. Really glad I asked here first instead of going back to my old mechanic. Could've ended up paying twice for repairs like others have mentioned. Thanks again for pointing me in the right direction! Will definitely keep this mechanic's number handy for future issues.