BMW M4 Cold Start Nightmare: ECU vs Battery Mystery

I had a similar issue with my 2015 BMW M3 last month. The symptoms were exactly like yours, difficult starts and occasional complete failures, particularly in cold weather. Initially thought it was the ECU too, but when I took it to the garage, it turned out to be corroded battery terminals and loose connections. The mechanic showed me how the cable connections had developed a whitish-green buildup, which was preventing proper current flow. They had to clean the terminals, apply anti-corrosion coating, and properly secure the connections. The main positive cable also needed replacement as it had some wear near the terminal end. It wasn't a major repair, but neglecting it could have led to more serious electrical issues. Since the fix, my M3 starts perfectly every time. Given your symptoms, I'd suggest checking the battery connections first before diving into more complex ECU diagnostics.

Thanks for sharing your experience! That's really helpful, as it does sound exactly like what I'm dealing with. It's good to know it might not be as serious as an ECU problem. If you don't mind me asking, can you remember roughly how much you ended up paying for the repair? Also, has everything been running smoothly since then, or have you noticed any other electrical quirks?

Hey again! Happy to update you. The repair was pretty reasonable, I paid 155€ for everything, including the replacement of the main cable and labor costs. It was such a relief that it wasn't something more serious (and expensive) like an ECU problem! And yes, I can definitely say the car has been running perfectly since the fix about a month ago. The engine control malfunction warning that used to pop up is completely gone, and the car starts right up every time, even on those chilly mornings! No more crossing my fingers when I turn the key. Haven't noticed any other electrical issues either. It's funny how something as simple as bad battery connections can cause such a headache, isn't it? Trust me, if your symptoms are like what I had, you'll probably be looking at a similar straightforward fix. My advice would be to get it checked out sooner rather than later, these things tend to get worse if you leave them.

Just wanted to update everyone, I took my car to get checked out today, and you were absolutely right! The mechanic found heavy corrosion on the battery terminals and a worn-out positive cable. The whole repair came to about 180€, which was such a relief compared to what I was expecting for ECU work. The interesting part was when the mechanic showed me the old connections, they were in much worse shape than I'd imagined. He cleaned everything up, replaced the necessary parts, and applied some protective coating to prevent future corrosion. The whole thing took less than two hours. Just drove home, and the car fired up instantly, no more of that hesitation or struggling! Really glad I asked here before jumping to conclusions about the ECU. Sometimes the simplest solution is the right one. Thanks again for pointing me in the right direction!