BMW M3 Cold Start Issues, ECU Problem or Something Else?

Hey there! I had a similar issue with my 2010 BMW M5 last winter. Initially thought it was something major with the ECU, but turned out to be much simpler. The battery connections were corroded and loose, causing poor electrical contact. Took it to my trusted workshop, and they cleaned the terminals, tightened all connections, and applied some protective coating. Total bill was around 155 Euro, and the starting issues disappeared completely. My sister has a BMW 335i and had similar symptoms, it was also battery-related. Before jumping to conclusions about the ECU, would be helpful to know: Do you hear any clicking sound when trying to start? Have you noticed dimming headlights? How old is your battery? Let me know these details, and I can probably help you more specifically with your M3!

Thanks for the input! My M3 is at 92209 KM and had its last service about 3 months ago. When trying to start, I do hear a clicking sound, and now that you mention it, I've noticed the headlights dimming slightly. The battery is actually quite old, probably around 4 years now. Didn't think about the battery being the culprit since everything else seems to work fine, but your experience makes a lot of sense. Might be worth checking the battery connections first before diving into more complex issues. Will get those terminals checked out!

Hey again! those symptoms you described are exactly what I experienced! The clicking sound and dimming headlights, that's totally textbook battery/connection issues. After dealing with my M5's similar problem, I'm even more convinced it's battery-related. Four years is actually getting up there for a battery, especially in a performance car like an M3. These cars are pretty sensitive to voltage fluctuations, and the computers need consistent power to work properly. When I replaced my battery after cleaning those connections, it was like having a completely different car, started up instantly even in the cold. Quick tip from my experience: while you're getting it checked, make sure they test the alternator too. My mechanic did that as a precaution, and it's good peace of mind to know the charging system is working properly. Also, if you do end up needing a new battery, definitely go with an OEM-spec one. I tried to save some cash with an aftermarket battery once, and it just wasn't worth the hassle. Keep us posted on what you find out! Pretty sure you'll be back to enjoying that M3 properly in no time.

Thanks so much for all the detailed advice! You've definitely convinced me about the battery situation. I just called my mechanic and scheduled an appointment for tomorrow morning. They'll check the battery, connections, and alternator as you suggested. You're right about going with OEM parts too, not worth risking issues with aftermarket stuff on these cars. Already learned that lesson the hard way with some other parts! It's actually a relief that it might just be a battery issue rather than something more serious with the ECU. I was worried I'd be looking at a massive repair bill! Will definitely update you once I get it sorted. Really appreciate you taking the time to share your experience, it's saved me a lot of worry and probably some money too. Thanks again!