BMW M2 Cold Start Mystery: ECU or Something Else?

I had a similar issue with my 2016 BMW M4 last winter. The starting problem turned out to be corroded battery terminals and loose connections. Initially, I also thought it was the ECU, but my mechanic found significant corrosion buildup at the battery connections during inspection. The fix involved cleaning the terminals, replacing the cable ends, and ensuring proper connection tightness. While it wasn't a major repair, leaving it unfixed could have led to more serious electrical issues down the line. The symptoms were exactly like yours, difficult starts, especially in cold weather. Before jumping to ECU conclusions, I'd suggest having your battery connections checked first. It's often overlooked but can cause these exact symptoms. My car's been starting perfectly since the fix.

Thanks for sharing your experience! This definitely sounds similar to what I'm dealing with. Would really help to know roughly how much the repair set you back? Also, has everything been running smoothly since then, or have you noticed any other electrical quirks pop up? Been a bit nervous about potential hidden issues with mine.

Hey there! Happy to help you out with those details. The repair was actually pretty reasonable, just paid 155 € to get those battery cable connections sorted out. Honestly, it was such a relief because I'd been worried it might be something way more expensive with the ECU. Since getting it fixed, my M4's been running like a champ, no more Engine Control Malfunction warnings or any of those annoying starting issues. That's been about 6 months now, and I haven't noticed any other electrical gremlins popping up. The mechanic did a thorough job cleaning up those connections and replacing the corroded bits. Don't stress too much about hidden issues, it really sounds like you're dealing with the same thing I had. Once those battery connections are properly sorted, you should be good to go. My advice would be to get it checked sooner rather than later though, just to prevent any potential damage to other electrical components.

Thanks everyone for the helpful advice! Just wanted to update you all on what happened with my M2. Took it to a different mechanic this time, and wow, what a different experience! Turns out my issue wasn't the ECU at all, had a failing starter motor and some nasty corrosion on the battery terminals. Total repair came to about 400 bucks, which wasn't too bad considering I was expecting much worse. The mechanic was super straight with me, showed me exactly what was wrong, and even gave me some tips to prevent future issues. Car's starting like a dream now, even in the cold. Such a relief it wasn't anything more serious! Really glad I asked here first instead of heading straight back to my old mechanic who probably would've charged me for unnecessary ECU work. Definitely learned my lesson about getting a second opinion when something feels off with the initial diagnosis. Thanks again for pointing me in the right direction!